It's always pretty cool finding something you don't see every day! I have seen more than a few systems with multiple WW's in different configurations, so while I would not call them rare, they are still pretty neat to find.
Not that common but I've found a few in my meanderings to date
It seems to me that 2 or sometimes more WW in a system is actually more common than just a single WW. But actual twinned ones as in they are in a binary system is fairly rare.