I perfer using a box with a divider. If one of your subs happen to be a little off, or one blows it will cause problems for the other sub. Not to mention I don't know of any advantages of running them both in the same enclosure.
The guy at the stereo shop told me that I would gain about 2-3db without the divider. I thought he was full of it. Thanks for clearing that up. He is also the same guy that built me a dual vented box that sounds like crap. $400 waisted cause now I'm back to my sealed box which sounds better. (not as loud though) Thanks.