You're talking about polyrhythms...very hard to learn as a drummer but a blast once you get comfortable with them!
Basically you start counting 4/4 (best way is to slap both knees) in quarter notes. Now continue counting 4 and speed up a bit until you get 5 equal hits in the 4 count. Your best bet is to learn 3 against 2(6/4 time) first, then move on to 5 against 4, 7 against 4, 9 against 4, etc. Once your comfortable with quarter notes try eighth notes, then sixteenth, triplets...
Generally polyrhythms are used in percussion and piano technique but there's no reason why you can't use 2 instruments to the same effect :)
Bill