We saw a heartbeat!
And a second sac!
Peanut A (who is currently known as The Showoff, because he/she popped up like a champ on every ultrasound angle) is measuring at 5w6d, with a nifty little heart rate of 140. Peanut B is measuring at 5w5d, but the RE wasn't sure if he could see a heartbeat or not. B definitely looked smaller, but I guess that could have been the angle.
The RE gave B a 35% shot at making it -- he said that in most cases he's seen like this, that's been the result. So oddly, our chances of having twins are pretty much the same as they were 3 weeks ago at our transfer. We should know for sure what's going on at our next ultrasound in 2 weeks.
I'm still processing all of this, but am so so happy we saw one heartbeat....it's more than I hoped for, frankly. I feel like I can't commit to being happy about twins, since things are still so up in the air. So I'll just hang with my Peanuts, feed them lots of good food, take them for walks and swims and acupuncture and pedicures, and wait.
Thanks for all of the super-supportive comments over the last few days....you all are the best.