Kazura wrote:

Scott, secondary bases are far more flexible I've found. http://wlodb.com/compounds/5037
That's one I know for sure works, I've done it myself quite a few times. Primary Bases MUST, to my knowledge, match. But secondary isn't as vital; just recommended to match for best results.

EDIT: Came across a confirmed compound. http://wlodb.com/compounds/4151
It does the same thing; primary base matches, secondary doesn't. But it's confirmed.

More Examples (done updating)
http://wlodb.com/compounds/1025
http://wlodb.com/compounds/5174
http://wlodb.com/compounds/4045
http://wlodb.com/compounds/4815
http://wlodb.com/compounds/4397
http://wlodb.com/compounds/4446 <- To show it doesn't just apply to 2 item compounds