Sunshine,
I suppose there is a few things going on here, it worked for you before so it can happen.
,at my clinic there i about a 10% difference in the chances of sucess once you get to ET stage from the Embies depending if they are fresh or frozen maybe ask the clinic what theirs is.
You should also ask yourself what would cause the most heartache
1. trying with 2 embies and getting another set of twins

.
2. trying with one embie at a time and seeing if it worked before going on to the next stage??
As it would not be a whole cycle there would not be as much drugs etc for the frozen cycle. I would seriously suggest that you make contact with the clinic counsellor over this one its not something that you can decide like that everyone would have a different answer.
Good luck with your decision. and you know what it worked for you before you have your lovely twins to prove it. What does your DH think?
Bro