I think you should stop listening to the guy at the petstore because your current setup is going to be extremely stressful on those snakes. You will likely have feeding problems no matter what method you use to feed them.

Don't worry though. A lot of us have been in your position before, where we received bad advice from a petstore and wasted money as a result.

Go with separate enclosures and make them smaller.

Also, how are you heating the tank? What are the temps like?