The myQ Connected Garage subscription is associated with a vehicle and not with the My Mitsubishi Connect account. However, you do not necessarily need to purchase the myQ Connected Garage for both vehicles.
When purchasing the myQ Connected Garage service, if you have 2 vehicles, then you must select the vehicle that you most want to associate with the myQ Connected Garage service.
Then you can control the myQ Connected Garage through the My Mitsubishi Connect App no matter which vehicle is selected.
Similarly, if you are in the vehicle and try to access myQ Connected Garage service from the infotainment screen via Android Auto or Apple CarPlay, you can manage your connected garage from either vehicle as long as you select the vehicle for which the myQ Connected Garage subscription was purchased.