If Im not mistaken, Rod Jenkins won the IBO world champshionship in 2001 with a compound and then won it with hislongbow in 2004.

It can be done.

I don't think there is any merit to what your shop owner says. They are 2 completely different things and if your brain wants to cross them up, you have a MUCH bigger problem on your hands. The only thing I could see is having a limited amount of practice time to try and be proficient with both. But if you have the time, it should be no issue. Shooting a compound isn't that difficult.

I try to make it a point never to take advise from behind the counter sales people. Unless he has the creditials to back it up. Has he won any sanctioned titles of any sorts in either discipline?