I have a 100 watt MV head. When I run two 8 ohm 2x12 cabs with it, I plug both cabs into the back of the head and switch the head to 4 ohms. This is per advice here and from the manufacturer. I used to run a 120 watt cab and a 60 watt cab. My understanding is that it's safe to do this with a 100 watt head because each cab is getting half the power. But, what about a 40 watt 2x12 G12M Greenback cab plus a 60 watt cab? There's a bit of wattage mismatch with the 40 watt cab. It is probably worth noting that with this particular head (Splawn) it's hard to get the master above 9:00-10:00 even live with drums.