I supposed you’d have to use the cue mix. You could also use the Cubase headphones as studio 1, and output as studio 2. Set up in control room.
But unless you mean studio 1 is a different room to studio 2, where you might want to have them both on at the same time or not, there is literally no reason to use both monitors at the same time in the same room. if I’m wrong please elaborate.
I use Cubase Headphones for the input record monitoring. Then the 4 outputs are A: Mix Headphones, B: Monitors 1, C: Monitors 2, D: Hifi
On my audio interface, the master output is already the same as headphones, meaning A had to be headphones not the monitors, otherwise selecting it would still play both and i’d have to turn off my monitors. The Cubase headphones is the 2nd interface.
You have to think about it and get to truly understand how your equipment can work.