hi J.,
There are a few Paci Fairy stories out there but quite honestly my daugther wasn't buying any of it - I found you cannot rationalize with a 2 1/2 yr old about giving up such an important possession and my daughter just wasn't able to plan ahead for such an event.
I finally explained to her that this was her last paci, (got rid of all extra ones) and pretty soon it wouldn't work so good anymore because it was getting worn out and paci would then need a long nap, and when that happened we would put it up in a safe place to keep for her.
I then cut a small hole in the tip of the paci (okay go ahead and flame me, probably not totally safe) be sure to watch that your twins don't get any 'chunks' from their broken paci. But they will not be interested in sucking on it that way and very soon notice that the paci no longer works and bring it to you to 'fix it' at which time you can remind them it's time to put the paci away for a long nap.
We put our daughter's paci away in a special little dish up in the cupboard, let her kiss it goodnight and 'tuck it in' and once in awhile she'd ask to 'visit her paci'. I explained she could not put it in her mouth but she could look at it and hold it and then it would need to go back to its special little bed. She loved doing this and found it very funny later on to see that faded old picture of Baby Big bird on the other side of the paci.
Of course the next time we were in the store she expected me to buy her a new one - but I told her that because she was almost 3, the store would not sell it to us. (again a fib, sorry but it worked).
She never cried for it once - and she was VERY attached to it!
good luck
W