I totally thought this with my b.g twins at 18 months old too- for all the same reasons.
My daughter would sit there forever though, entil she went- my son went maybe a handful of times, if that, but mainly would sit there for 45 minutes then get up and walk 5 feet and pee where he stood. My envisioned goal of them getting potty trained together was scrapped-
My daughter, at 2 1/2 suddenly was overnight completely potty trained. The diapers at night only lasted a few months (pull-ups at night were called "panty-diapers") and now, she will be 3 next week and sleeps in panties.
I knew she was completely potty trained overnight when she woke up that morning, went to potty, and then went outside to play later and asked to come back in to pee.. nice! She KNEW when she needed to go and could hold it til she got there- that is the key I figured out! She does #1 and #2 in the potty, always. She is already climbing on the big potty by herself, no stool, no seat thing.
NOW- my dear dear son.. will be 3 next week, and still doesnt care a thing about the potty. He doesn't understand the feeling of when he needs to go at all, and will still pee in the floor if left with nothing on.
On a postive note though, when my dd decided she was potty trained, he went in a completely different "brain direction:" and suddenly was a VERY fluent talker! She is still hard to understand a lot, but he translates for me! LOL!
Their brains focus on one developmental thing at a time, I have learned, and I don't know about you but MY b/g twins are EXACT OPPOSITES of each other in every feasible way!
Email me if you want more- I have written a book here!