so can two modifiers be used at the same time?
Sure, but some combinations yield funny results, because they can't logically work together.
So is this a wishlist for v9? Or can it be added later when a patch comes out for 8.
The only consideration is performance. Something like (rock ~%=50) (folk ~%=50) requires two complete searches, and a post process phase. Right now, all searches are one search and a post process phase. Anyway, let us think about it...
And you can sort of do what you want now with a playlist group with some smartlists in it. You just need to do a little of the math to convert from % to # of tracks.
Group
---Smartlist 1: rock ~n=20
---Smartlist 2: folk ~n=20
Now the group will list 40 random tracks, 20 from each genre. Hit [F5] to rerun the search and get a new 40 tracks.
Take care.
-Matt