2nd u/s today. Twins despite SET!

So for those of you who are also on the New FSH board you might remember that I agonized over doing a single embryo transfer because I didn't want twins. I ended up doing the SET and feeling pretty good about it. I'm 6 weeks 1 day now and today I saw not one heartbeat but two! Apparently, the little bean split. What are the chances! It was such a shock! The first one is healthy had a strong heartbeat which we could even hear! The second one looked to be about a week behind and had an enlarged yolk sac so my dr said that it probably wouldn't make it. I know it's pretty common to have a disappearing twin. It's also nice to know so I don't freak out completely if I see blood. OMG though, what a crazy ride this is. I'm still shaking. Anyone else have this happen?