I have a throttle quadrant. I use dual throttles on all twins, and 4 on the heavy bombers.
You cannot use 2 throttles to control 4 engines. Throt1 will controll eng1, but throt2 will control eng2 (both on the same wing!), but all the unassigned engines (eng3 and eng4) will also be assigned to throt2, I believe (one of the others, I can't recall which).
What you have to do is set up a flight mode. Whenever you fly a 4-engine bomber use that mode. Map ONLY throttle1 in this mode, and whatever you want to the other control (say, RPM, trim, whatever). Then when flying 4-eng bombers you just use the 1 throttle. Then go back to mode1 when you fly twin-eng or single-eng bombers/fighters.
Make sure to map a button or key to "mode1" and "mode4" so you can get back and forth between the modes when you want.
As for mossies: Not much benefit from using 2 throttles, other than the "this is sweet" factor. You don't want to throttle one back in a turn or you increase risk of a spin into the inner wing. Better to use rudder during turns than throttle.