S.Y.
Ahhh, I remember that willfull age (it was a long time ago but is also coming up again for me soon) :)
Sounds like they are asserting their independence and it would be just a power struggle to try to continue. I concurr with the other posts...take a break and revisit the issue. I am sure it will be disapointing, as you were sooo close! Argh! Ah, well...that's one thing that we ultimately can't "make" out kids do. Frustrating.
How about this? When the time is right, have some special "potty books" that are especially for when they sit on the potty (my ex-husband is my inspiration here lol). I do the same when I change my dd's diaper to avoid the dreaded "horizontal twist dance" she does when she is bored and wanting to get up and play. She isn't alowed to play with them any other time, ONLY when she gets her diaper changed. That way they have intruige...your twins can pick out the "coolest" potty books (or a toy they can play sitting down)and then they may look forward to getting on that ol' potty so they can read/play with them.
Hope this helps! I think you are doing a great job, Mommy! :)