Do you really need that large a unit?
Based on your friend's report, it sounds like, if anything, your unit will be too large. You're doing it for medical reasons not comfort, so you say.
Given the ever-rising SEER numbers (efficiency) we're seeing from consumer tier units, you may be better off simply seeking a 120V unit.
Those SEER numbers reflect how much you will be paying for electricity so it's a very big deal.
Definitely do not take a "hand-me-down" unit without looking closely at SEER numbers. Many "gift / side-of-road / hand-me-down" A/C units are older, and did not benefit from the very rapid gains in A/C efficiency made in recent years. A typical 8000+ BTU unit these days only needs about 5 amps @ 120V. You'd have no trouble running 2 on a single 120V/20A circuit.
240V units are largely gone by the wayside; they don't need the energy anymore... the ones that remain mostly support 240V because they are needed for installations already wired for 240V.
Lastly, if you consider a "portable" air conditioner (the kind with a hose that goes out a window, it's very important that you consider a 2-hose one. 2 hoses seems like an anti-feature until you learn how they work.
From a comment in chat:
I know the whole thing is arithmetically wrong, I have 50 coming in, and I want 50+20 out, so that is why I don't want to run both devices at the same time, at least not at capacity
... and that's the biggest worry you have? Not
the comical cavalcade of Code violations...
It seems like you want to "freestyle" safety based on your lay impressions of electrical safety. The entire point of Code is to pass down the paid-for-in-blood experience of a century of electrical use, so you don't have to pay in blood for a lesson already learned.
That is especially relevant in a condo, when your electrical fire could kill innocents in other units. I guarantee there's a city ordinance, or HOA rule, that says you must use a licensed electrician for work in the unit. Weren't we just talking about new-air-conditioner finances, and "good money after bad"?
The use of a circuit breaker is a good sentiment, but you can't just use some random loose circuit breaker in some random enclosure. You must use approved equipment and enclosures (NEC 110.2) and install it according to its instructions (NEC 110.3) in a neat and workmanlike manner (NEC 110.12) and with appropriate working space (NEC 110.26). And this is just the first few pages of NEC.
The oven/range circuit must be dedicated to oven/range. You are only allowed to have one load/outlet on a 50A breaker (NEC 210.23) unless it's one of the listed exceptions (ranges/ovens covered in NEC 210.23(C)). An air conditioner is not cooking equipment and is not allowed under the NEC 210.23(C) exception.
Further, your range is already too large for a 50A circuit. It's only on one because of a Code exception (NEC 220.55) which factors for the fact that not all burners are at 100% duty cycle all the time. When you add non-range equipment to the circuit, you void that discount and now your circuit is way too small.
Your logic, "all my burners are not 100% all the time" is correct but Code has already factored for that - there's no headroom there.
A 50A circuit can only have 50A receptacles on it. NEC 210.21(B)(3). So there's no such thing as tapping a 50A circuit to a 15/20A receptacle.
Even when you're in industrial spaces where you can use the tap rules to full effect, you're still talking about hardwired loads.
And I haven't even started into Article 440, which is the section applicable to air conditioners. I'm sure plenty of violations are there too, but no point beating a dead horse.
You're gonna spend more on Polaris connectors than a subpanel would cost!
So it's simple enough. Use a subpanel.
You currently have a 50A branch circuit from main panel to range. Route that to a subpanel, at which point it is a 50A feeder. You then install 2 breakers on the subpanel, a 50A and a 15A.
Now we sidestep all the above Code violations, because we're no longer torturing a branch circuit.
We now have 2 individual branch circuits, each following its own rules - the range keeps its favorable derate, and the 15A A/C circuit is definitely allowed a 15A receptacle.
As far as the "50A+15A > 50A" concern you have, the 50A feeder supply breaker will take care of that.
Do you really need a 50A breaker in the panel when the panel is fed from a 50A breaker? Yes. The supply line is feeder. You can't put loads on a feeder, and a breaker is ten bucks, so who cares?
Now we just need to work out the Article 110 issues about approved equipment and working space. Use the smallest subpanel you can find - a 2-space panel will suffice with a 15/50 quadplex breaker in it. Run of the mill Eaton, Siemens etc. stuff from the hardware store will satisfy NEC 110.2. Install it competently, and we tick off 110.3(B).
The real nut is the working space - you need 30" wide x 36" deep x 78" high working space in front of the panel. Surely there are either kitchen cabinets next to this oven. I would hack a kitchen cabinet's shelves so there is room for the subpanel to sit right at the front of the cabinet facing outward. It will block some of the cabinet's space, making it inconvenient to put things on the shelves, but this is the best we can do. So you open the cabinet door and voilà, there is the face of the subpanel right there. Presumably you walk around that area a lot, so there is 30x36 working space floor to ceiling, should be all set.
The panel might be inset maybe an inch from the face of the cabinet doors... an inspector might flag that, but I doubt it. Putting subpanels behind cabinet doors for aesthetics is perfectly common.