i havent done it,but ive done other things where im combining an accapella thats in one key and tempo and a beat that has its own key and tempo. if your just gonna sample hits quick riffs etc, you can just chop it and put them in place as long as the tempos are not extremely different, nad even then if you slide the sample around enough you might get lucky and itll still sound good. but if youre wonder how to do this with loops, first learn the BPM's and keys of the songs, then slow one down or speed one up to match the tempo of the other, then add a
pitch shift (pitch shift is changing the pitch of the song without changing the speed)to one of the tracks to match the key. u can do this in any order (first match the keys by changing the speed of one track till its in the same scale,then add a "time-strech"{time stretching is when you change the speed of the song without changing the key} to get it to the same BPM of the other song . etc