Lise - curiosity got the better of me and it seems I was half right. Multinucleation does refer to two nuclus' but it has nothing to do with twins. I found this on advancedfertility.com if anyone else is interested.

Presence of multinucleation

An embryo is multinucleated if more than one nucleus can be seen in any individual cell on either day 2 or day 3. After day 3 it is extremely difficult to identify the presence of multinucleation. The majority of multinucleated embryos have been shown to be chromosomally abnormal in PGD studies.

However, sometimes multinucleated embryos implant and lead to a healthy pregnancy and birth of a normal baby. These embryos are generally only transferred to the uterus if they are the only ones available.