How can I connect two indoor ductless A/C's to one compressor?
If his house is relatively new and well insulated and not too enormous (1500 SF or less) and not in a very hot area, two tons is probably enough for the whole house most of the time. I've got 2.25 tons downstairs and 2.5 tons upstairs in a 3200 SF house and it's always been more than enough so 2 tons for 1500 SF or less is probably fine even if you run both evaporators at once.

However, I would want to be sure that you could switch off one of the two evaporators because that seems essential to his planned usage. It could be for energy savings or just because he already knows 2 tons isn't enough for the entire house, he doesn't say.

The actual question of connecting two evaporators to one condensor is quite easy, you just need to add a T to the freon supply and a T to the freon return. But how will the compressor be told to turn on? You don't want the room with the evaporator you are not using to run the compressor.

And how do you stop freon from going to the evaporator you are not using?