I have an insulated garage and bonus room above it. Can a HVAC feed both the bonus room and garage, but the bonus room have the air return and Tstat? The bonus room is accessible from inside the house, so there would be no "flow through" between the garage and bonus room. I'm not too concerned about the garage being at different temperatures than the bonus room, but want to insure there is not some issue with having two rooms with no shared air space, but the return in only one room.
You'll need to use a multi-split for this
The only way you can get this to happen is to use a multi-split (multi-zone mini-split), as IRC M1601.6 prohibits ducts that supply air to living spaces from also supplying air to or returning air from a garage, for obvious CO and fire hazard control reasons:
M1601.6 Independent garage HVAC systems
Furnaces and air handlers that supply air to living spaces shall not supply air to or return air from a garage.
Since multi-splits have independent air handlers for each zone, though, piping only refrigerant back and forth, this lets you use one outdoor unit with two indoor units, one for each space, and gets you what you most likely want for this, although the controls for the indoor units will be separate.
There are multiple ways to do this but since it is a retrofit a 2 head (or more) mini split system makes the most sense. With a mini split system each room has its own control. There is no ductwork check them out. the air in one room can be totally isolated from the other room.
With a conventional air handler the rooms can each have there own intake and output ducts but the air for the 2 rooms is mixed at the handler.
The ductwork gobbles up quite a bit of space and the controls can be in one room but if the rooms are not well balanced the controls will feel inadequate.
Look into a mini split I have multiple zones and they are quite efficient, if you are not using one room turn it off the compressor unit slows down and saves energy. The pipes and power all fit through a hole in the wall for each room, if the rooms are large or 1 is, that one can have multiple heads if needed and the compressor is sized according.
If you want a central air add on it may cost much more than a mini and take up more space because of the ductwork. The temp control will be better with a mini each room being its own zone. The longer I have my mini split system the more I like it the efficiency is outstanding and comfort is a push of a hand held remote away.