there is no reason to vent out heat when you have a a/c capable of handling it all....
Here is how I'd build it
have two seperate sealed and insulated rooms with either a dual zone mini split or 2 separate mini splits or window units. Then have a huge fan on a thermostat that vents the heat out of the basement. The external parts of the window units/minisplit will be in the basement.
this will work great in the winter time when it gets too cold for the outside unit of your mini split, and when it warms up you'll probably need to move the mini split outside, because it will probably get too hot down there during the day.
It really depends on your climate and how insulated the area is.
I'm in the south and it's never really cold down here, so I have to use a lot of a/c to get things cool. A cold night for us is to touch the freezing point.
Your "no" a/c on 4k is with active venting of fresh air. Once you seal the rooms, the only way to remove the heat is through the a/c. Unless you have vented hoods and use outside air to run through the hoods and have them exhaust outside the room.
Well, for example, a mini-split rated at 24k btu with 2 zones can only remove 12k btu's per zone. So if both zones are running the exterior unit is capable of maxing out both zones.
Now you might could custom order one with 2x 24k btu indoor units and connect both of them to one 24k btu exterior unit. It should work just fine as long as both of them aren't running at the same time, and even if they did, they still would have a 12k btu's of cooling but they would lose all capabilities of dehumidifing due to technical reasons.
technical reasons = a 24k btu coil provided with only 12k btu of cooling and a fan moving air at a rate of speed appropriate for a 24k btu coil will not allow condensate to form on the coils. reason being, water forms on the coils when the air surrounding it reaches the dewpoint, if there is too much airflow, the temperature of the air surrounding the coils will never reach the dewpoint, and thus, no formation of condensate. But this is, only if both 24k btu indoor units are running at the same time connected to only one 24k btu external unit.
I don't really see the benefits of having 6kw on a flip flop vs just having one 12kw room. Maybe if your panel is only 100amp, there would be some benefits, but for a 200 amp panel, the need for a flip flop doesn't really come into play until you go over 12. These are assuming your living in the same place, if not you can go a bit higher.