We are pulling an old Jenn-Air range and replacing with a separate Jenn-Air stovetop and oven. Due to the downdraft they cannot be placed on top of one and other but will be in adjacent cabinets. The current electrical setup for the range is a double 50 Amp breaker with an 8 gauge wire that supplies the unit. Since this is in an island set into a concrete slab there is no way to run new electrical lines. The new oven is 3.6KW and the stove is 7.0KW. The minimum circuit and wire sizes are 20 Amp/12 AWG and 40 Amp/8 AWG according to the installation manuals. Can we run a 12 gauge wire off the existing 8 gauge wire to serve the oven and keep them on the same circuit? I will have an electrician make the connections, but for planning purposes I need to know whether this electrical setup will be OK.
Blame whoever buried that cable in the slab for your predicament
Since some nitwit thought burying a /2 SE cable in a slab was the best thing since sliced bread, your project is now in a predicament due to the lack of separate neutral and ground wires in the cable. This missing wire problem forbids you from fitting a subpanel here (as you'd need a separate ground for that), and it also forbids you from running further cables from here due to issues with the NEC 250.148 requirements for terminating EGCs conflicting with the Exception to NEC 250.140 that permits continued use of existing 3-wire range circuits.
This leaves you with only two ways out: the destructive way and the not-so-residential way. The destructive way would be to dig the culprit cable out of the slab and replace it with something less awkward, such as a 1" ENT (smurf tube) that can then have individual THHN wires run down it. This provides plenty of expansion space, and uses methods and materials that most residential electricians are familiar with, but it sounds like the level of physical demolition and remodeling (busting and re-doing floor tile and concrete slab, ripping out and replacing island parts, and opening up wall cavities to route the ENT out of the slab and back to the panel) this requires makes it a non-option in your case. (It'd also take quite a bit of money as a result of all that time and work.)
It's transformer time!
However, there is a savior for your situation, riding in from the bowels of high-rises and the floors of factories. While not normally done, nothing in the NEC prohibits the use of dry-type distribution transformers in residential work, and they have certain properties that make their use advantageous and cost-effective in some circumstances. In particular, since a distribution transformer's primary and secondary are coupled only by their mutual magnetic field, it provides electrical isolation. As a result of this isolation property, the neutral and bonding systems effectively "start over" on the secondary side of a transformer, yielding something called a separately derived system in the NEC.
What this means for you in practical terms is that your 3-wire cable can feed a suitable distribution transformer, and then that transformer can feed the cooktop and oven in turn via a (small) breaker panel, thus providing a fully Code-compliant, 4-wire feed to the new appliances without the need for extensive demolition and remodeling work. However, there are some caveats to this approach, so read on before you make your final decision here.
First off, transformers are not like most other pieces of hardware in a house in that they are dense compared to a typical appliance. A transformer of the correct size and type for your application, namely a 10kVA, single phase, 480/240V primary, 240/120V secondary, enclosed, dry-type transformer as a 7.5kVA unit is just a scotch too small for a range circuit, will fit into a space 2' high, by 15" wide at most, by 12" deep, yet weighs over 150 pounds! This, along with the fact that transformers are healthy space-warmers that need cooling air in order to have a long and productive life, means that you can't simply hang one on the back of an average kitchen cabinet.
Furthermore, we'll need to fit a small breaker panel into the island as well in order for this approach to work. This provides a home for the new neutral-ground bond required by the transformer's presence, as well as protection for the wires on the secondary side of the transformer. Fortunately, we don't need much here; a "spa panel" sized breaker box flush-mounted into the access hatchway to the transformer compartment will suffice. Note that this access hatchway will need to be louvered or grilled in order to provide cooling airflow for the transformer, and will also need to be proud of the cabinet front to keep the countertop lip from infringing on the 110.26(A) clear working space for the spa box. The transformer itself does not need to have a full clear working space provided for it, though, as NEC 450.13(B) permits an installation into a hollow space such as the interior of a cabinet.
Finally, the transformer itself is something that you or your electrician will need to source through a local electrical supply house (or used, via say Craigslist in your area); common pricing as of the time of this writing is somewhere in the $500-$600 range. They are too heavy to be shipped as parcels (and require freight shipping service as a result), and also are simply not carried by big-box stores as they are rarely used in residential construction. As a corollary to this, that last fact also means that you will also need to make sure your electrician of choice is familiar with transformer connections, as many residential electricians never encounter them in the course of their work.
Implementing this, industrial style
The way this works out is that you'll have to build a sturdy structure inside the cabinet bay where the old range outlet was to mount the transformer on as it has to be mounted upright in order for the cooling to work out correctly; this structure will need to be anchor-bolted directly to the slab instead of being attached to cabinet components, as well, due to the aforementioned weight issue. From that existing box location, the installer can run a 3/4" LFMC whip with 2 6AWG stranded THHN hots and an 8AWG stranded bare copper ground in it to the transformer; this connects to the primary side of the transformer, which is strapped for 240VAC use with the hots connected to H1/H3 and H2/H4 at the the transformer and ground connected to the transformer's grounding lug. Note that this may require replacing the existing junction box with a larger box: a 4-11/16" square box provides enough room here to connect to the existing cable. Also, the split bolt that was used for the original neutral/ground connection needs to be removed and replaced with a compression connection as this equipment grounding conductor is doing double duty as a grounding electrode conductor as per the Exception to 250.121.
The transformer's secondary then is wired to the back of the spa-box-sized loadcenter using individual wires in a raceway; a 3/4" LFMC whip with 3 6AWG stranded THHN wires for the hots (aka X1 and X4) + neutral (X2/X3 taken together) and an 8 AWG stranded bare copper ground will do the job nicely here, although at the transformer the secondary ground wire needs to be connected to the primary ground wire with a compression tap connector in order for the grounding electrode path to meet the splicing requirements in NEC 250.64(C) point 1. Into the breaker box goes a 2-pole, 50A breaker of the correct type for the new panel, with 6AWG pigtails from the breaker hots to an 8/2 NM cable to the cooktop and a 12/3 NM cable to the oven. Note that while a separate ground bar is a good idea here, this panel must have its bonding screw or strap fitted as if it were a main panel; this establishes the new neutral-ground bonding point required by the transformer's presence.
Last but not least, the existing 50A range circuit breaker in the main panel will need a lockoff device fitted to it to meet the requirements of 450.14 for a lockable disconnecting means for the transformer. For your main panel, this means that your installer will have to buy either a GE THP100 (for full sized breakers), or a GE TQPPL (for half-width/double-stuff/thin breakers) and fit it to the range breaker in the main panel.