I've got a whole slew (8-10) of the "Galaxy Metal Gear Box" USB drive enclosures -- never had a problem with any of them and they were pretty inexpensive, but it looks like they're no longer made...?
A clue for shopping on Newegg:
http://www.newegg.com/Store/SubCateg...s&Order=RATING
Sort by rating... 5 eggs and 300+ ratings? You're probably going to be okay.
ETA: I've never heard of an external USB drive where only one was allowed on a system at a time. That sounds more like a problem with the system than the drive, but who knows? USB operates under Plug and Play rules and two identical devices will present identical vendor and product ID's to the system -- it's up to the system to keep them separate. I think...