You can put different kits into different style divisions, but you can't combine elements from two different kits at the same time. There is only the one drum track. If a sound is only available in the one kit, you can't have it sound while a second kit does the main rhythm.
Every Roland kit contains quite a few 'alternate' sounds, primarily kicks and snares, and the Makeup Tools section allows you to easily change out these rather than the pre-programmed sound. But is by no means comprehensive. Each kit basically only has the one set of hihats, toms, crashes, etc.. There is no way to take, for instance, the hihats of one kit and combine them with the rest of the kit from another. And certainly no way to store these edits (when you CAN make them), other than in the ONE style you are working on.
You certainly don't get the ability to create your own 'custom' kit out of ALL the arranger's available sounds, and you can't run two drum tracks simultaneously, only one at a time. Yes, you aren't limited to one kit for the whole style, but there are still significant limitations to what you CAN do....
If you have found some kind of 'back door' way to combine say the hihats from one kit with the drum sounds of another, I would be VERY interested in hearing how...