You need 2 separate circuits, one for the air handler and one for the condenser, an air handler calling for 50 amp breaker means that they're installing an 8kw back up heater.
Service disconnects dont have to have a breaker, most often they just install a pull plug which is just an on/off switch. You have to run #8 wire to support the amperage for each unit.
My old system was like that, the new ones were different, far more efficient.
That's why I said he/we need to have details of exactly what he's installing before giving exact advice.