You would think that every one of the pups in that litter are technically twins( or triplets, or quads etc.), maybe not identical, but fraternal. I thought I had twins when Rosie whelped, 2 pups came out within a minute of each other, they had the same exact chest marking, but one was a boy and the other was a girl, so they were not twins. I don't think they will miss that twin any more than the rest of their littermates, they don't see anything for weeks, so they can't really know which one is which.
Interresting though.