In a live performance, it is unlikely you will have any stereo instruments with the exception of those that produce stereo (keyboards, HD500, etc) and they will also use two inputs. We had a similar discussion in another thread on here. You wont get a true stereo, but, you can pan things to left and right using 2 monitor sends. You won't be able to do any panning with a signal coming out of the monitor sends (a,b,c,d) from the M20D. You will have to use two sends. If you want something in both ears equally (vocals?), then you must send the same amount of signal to each ear. (from each send). Then you can put guitar over in one ear by increasing guitar in send in "A", and decreasing guitar in send "B". And bass over in the other ear by increasing bass in send "B" and decreasing bass in send "A".
I am not overly familiar with IEM, but if they have a pan on them, you will still likely have to adjust at the board, and leave your IEM pan in the middle. Assuming it just pans send A and send B. But the first paragraph here is the only way I think you will be able to pan things or keep them in the middle on the M20D as far as a monitor mix goes. You can do it on the front end through the PAN knob, but not on the monitors. Just not built that way.
(and then I read the post above and realised it is a long winded repeat of that...more or less...sorry lites...)