How about Fred and Wilma Flintstone. Here's the costumes.
http://www.joke.co.uk/showPart_large.asp?part=60995
If not them check out the rest of the site they have a whole section on costumes for couples. It's a really good site and very fast delivery, i ordered a costume from them at 7pm last thursday night and it was on the door step 8am Saturday morning.