Good question?
If I understand your question right, your looking to change out your control handle so that you can shift to reverse by individual engine, rather than both at one time. If I'm correct in this, then I don't think so. You may be able to find some aftermarket company that may build this, but then you'd have to go around the neutral safety switch and the rev limiter so as not to over rev while one engine is in neutral. The principal, like on some of the big boats, will let you "walk the dog" in ship talk so to speak, sideways to get pierside or do a "U turn" or what ever. It would be nice if Sea Doo offered the choice of controlling the engines individually........that would be so cool.
I use babbittsonline.com to get my parts and I checked their aftermarket stuff and they didn't have anything like it.....hope you find what your looking for!