I have a G6 case and obviously the current draw of all the modules is
less than the 1.2A max of the psu2. In fact it is currently way less
than 400ma. Can I safely add another 6U case and connect it to the
PSU2 via doepfer bus boards or is one 3U more enough

I heard that as you draw too much power from the supply the total
noise can increase.