The first question is are they interested in it? I don't have twins, but I do have a 4 yo boy and 3 yo girl. It is very hard work potty training two kids at the same time. My daughter is completely potty trained, and my son is still working on pooping in the potty. When they started wanting to potty on the toilet...sitting on it, not doing anything and then flushing, I started them on pull-ups. That was a mistake...they looked at them like diapers. When I finally figured that out, I just put them both in big-kids underwear. That has help...granted I have to wash a lot of little boy's underwear since he has poopy accidents, but he is making more of an effort as he doesn't like the feel of it, nor does he like the fact that his sister is doing better than he is.
Just make sure they are showing an interest in the potty. Try to take them every hour. Take them together. Praise and make a big deal. Do they like stickers or anything like that? You can make a little chart for them and give them stickers for each time they do a tinkle or poopie. A friend of mine taped paper to the back of the toilet, turned her son around and let him color while he was waiting to let the poopie out. That is the hardest time I've had so far is keeping Tristan on the toilet long enough to let all the poopies out.
Good luck! They will get it.