Thanks for the helpful replies, folks. There would be no risk of doubling up on uploads from me, as I would have my files in separate folders, e.g., spoken word in one WinMX, and comedy and nostalgia in the other WinMX. I would also allow only one upload per user per MX, and only two users per MX, making a maximum possible of four files being uploaded from me at any one time.
I'm still in two minds about doing this; I don't know how badly it may use up my resources, and anyone Browsing me at present can see my whole collection and maybe see some other file they hadn't thought of getting before. If I run two MX progs then people won't see everything that I have to offer. I'm on WinMX mostly to share; there's not much that I want that is available from others (what I want is rare) but I hope to share my stuff around (what I share is rare, too).
Anyway, thanks to everyone for your help.