First, consult the installation instructions for each of the appliances. Those instructions are law. Literally, your state legislature has incorporated NEC into state law, and NEC 110.3 absolutely requires following installation instructions. Because, UL/CSA only tested it in those conditions, so the UL listing depends on proper installation.
It is not uncommon to have one single circuit supplying both range and oven. Code allows this and so do many appliance manufacturers.
Regardless, a #6 cable is only good for 55A@60°C, 65A@75C, and 75A@90C, (NEC table 310.15(B)(16). Generally you are required to use the 60C column for residential branch circuits (you can bump from 55A to 60A because you're allowed to round up to the nearest offered breaker size). If you want to go higher, you'd need to find a specific exception in Code, and then argue that with your local authority having jurisdiction (city inspector), and then he decides. I'm at the limits of my ability to advise; I wouldn't expect to get away with 70A on #6.
If the appliance instructions require (or if you prefer) a separate circuit and cable for each appliance, then I would retain the #6 (it's overkill for a 40A circuit but is a nice fallback if you ever install a 50A or 60A range/oven) and add a #8 or #10 to handle the other circuit. You are allowed to use oversized wires. If the #6 is a 3-wire ungrounded circuit, then I would definitely lay a parallel 10/3 or 8/3 - and use its ground to properly ground the obsolete #6, so it is now a 4-wire grounded circuit. This is allowed in ground retrofit rules: grounds can be shared, so you now have two 4-wire circuits on 7 wires.