I'm a mom of twin boys too. We didn't use two potty chairs - I had one, but my boys did better with a (padded) kid seat on the big toilet and a stepstool to help them get up there. We had the little potty next to the big potty, but they would fight over who got to use the "Big potty" versus the "baby potty". Mine did much better when we went one twin at a time - they tended to distract one another a lot. I kept the little potty next to the big potty for about 6 months for dual use, but it was only used 2-3x total once I got a kid-sized seat for the big potty.
The other advantage of using a kiddy seat on a big potty is they didn't have any issues with using a big potty when we were away from home. I had a foldable plastic seat that fit in my purse and we were ready to go anywhere. My godchild on the other hand HAD to have her "little potty" and refused to use a big toilet for a long time. Her parents had to drag around a little potty (& deal with cleaning it!)all over creation for nearly a year.
One trick that helped with the big toilet when they were begining, was completely taking off their pants/underwear, instead of having it around their ankles, which tended to trip them up getting up and down. Let them run around naked or with t-shirts only, as much as possible. Mine had only a handful of accidents when their bottoms were naked - almost all the accidents happened when they were fully clothed.
No pull-ups! One of the teachers at my son's daycare (on Univ. campus) was doing a masters in Early Childhood education and toilet training was her thesis topic. Pull-ups significantly extends the amount of time for kids to get fully potty trained, and she found the same thing in her research project. After we had used up all the diapers in the house, we used pull-ups occasionally when we had a long day of running around, long car trips, or some other occasion where an accident would have been a big disruption or especially embarassing for the child (i.e Thanksgiving @ relative's home). One twin was kind of lazy about getting to the toilet, if he had pull-ups on and we eventually had to make a separate reward for keeping the pull-up dry. This was the same twin who at one point decided he wanted to go back to diapers for a while. A couple of weeks later when his brother got promoted up in Day care to the (non-diaper wearing) "Dolphins" class, he finally got serious about the potty training. I don't purposefully have the twins compete with each other, but it turned out to be the most effective motivator for them. Peer pressure is nothing compared to "twin pressure"!
My twin who was the first to (daytime) potty train, on the other hand had problems staying dry at night for nearly a year, even with waking him up for an extra late-night trip to the potty. He HATED having to put on a diaper, but would use a pull-ups at night when we were away from home.