U.S. NEC on adding wires in a finished ceiling
I guess the first question would be if you have a engineered truss system or ceiling joists; they aren't the same thing (as I've learned).

Is there livable area on top of the garage or an attic/loft?

If they are just ceiling joists you can drill a hole right through the middle of the joists (don't cut out notches). I've done 3/4 inch holes with an auger bit; it would fit two 12/2 wires nicely. Try to keep the holes near the middle.