Hi M.,
I had the same idea. My boys are 18 months apart and I thought that it would be easier to train 2 at once. That is not what they had in mind!! My older son is very bright and told me, "I know how to do it, but I don't want to." And he meant it. When he was ready, he told me I could throw the diapers away because he was done, and he meant that, too. (It might have had something to do with me telling him that he wasn't having his birthday at Chuck E. Cheese in a pull-up, but I'm not sure.)
My younger son would stay dry in a pull up for days (9 months after my older son trained), but if I put big boys pants on him, he would wet them within an hour -- every time! It wasn't until her got a really bad heat rash while we were on vacation (and even though I knew it wouldn't solve the problem, I told him he would have to change back to diapers) that he finished training.
My older son finished training the week before his 4th birthday. My younger son trained just before he turned 3 1/2.
Be patient and they'll do it when they are ready or before they start Kindergarten, which ever comes first.!P Good luck!!