They can both run in a/c down to the same point.(about 50) They will run at colder temps, But you run the risk of liquid getting back to comp unless you have a low ambient control installed. Carrier makes a nice one for those units.
If you have two a/c units you'll need a seperate indoor unit, It could be an air handler or furnace.
Keep us posted on progress. I'm always curios what option people chose in situations like this.