Twin ET calves have been discussed here at least a few times that I know about. It isn't common, but apparently it CAN happen and does. There may be more possibilities, though, and every scenario might be a little different.
I asked the embryologist we use to do what little ET work we've done about this subject, and according to him, an embryo CAN split after it's transferred into the recip. He says he has noticed embryos under the microscope before and made a note that it might be apt to split, and then later one of those did result in twins. Some of them may not result in a pregnancy...I'm not sure. Other possible scenarios might be that the recip was bred naturally and has a set of her own twins, that she was bred and conceived and then an egg was implanted that also resulted in a live calf--meaning two calves out of two separate matings, so not actually "twins"--and then there's the possibility that two embryos were in one straw and implanted at the same time, however, he says this would take an extremely "incompetent" tech to do that.
