J.H.
They are just called stroller connectors (made by a company called Prince Lionheart).
Here's a link to them on amazon.com
http://www.amazon.com/Prince-Lionheart-6550-Stroller-Conn...
I think a friend got mine at BabiesRUs.
They are great...you do need to have two of the same umbrella stoller though. Or at least 2 that are made the same - the tubing frame needs to match up so when they are connected it is all symmetrical.