Make then break electrical switch?
I am a bit confused by the question - I think you're saying there's a 40 amp panel with a 20 amp two pole breaker for an air conditioner you want to add a 30 amp breaker for a washer dryer unit, and you want to make sure the washer breaker and the air conditioner breaker don't go on at the same time.  (Also, are you thinking break before make, and have it reversed in the question's title?)

Would it work to use a 30 amp DPDT switch on a 30 amp breaker, one throw going to the washer dryer, the other going to a disconnect with a 20 amp breaker feeding the existing air conditioner?  For example this one

Hubbell 1388

looks at a glance like it would be adequate.  

The AC and it's existing wiring probably has to be on a 20 amp breaker, so you couldn't just connect it to the switch. I think you could find a 20 amp disconnect to go between the AC throw of the switch and the existing wiring.  

Is there another panel for the apartment?  With all this on 40 amp main I'd see a lot of trips.