Hey J.,
My name is M.. I am actually a preschool teacher and have worked with 2 yr olds for 12 and a half years. I don't know EVERYTHING there is to know about potty training but i guess you can say i know more than alot of people because i deal with it on an EVERY DAY basis.
The 1st thing you need to remember is every child is different. Even though they are twins they are very different. every child learns at their own pace. There is no certain age that they are just going to start going potty and you can't just expect them to learn at the same time (I don't think you are but I usually tell the parents in our center that as well)
The 2nd thing you need to do is pay attention to them and REALLY see if they are interested in going potty.... if they don't have interest in going it's going to put alot of pressure on you both if you try to force it. ALOT of parents think "oh my child is 2 they should be potty trained before they turn 3" that is one of the worst things you can do is force your child to go when they are not ready. I am not saying don't try but hey if it's becoming a struggle for you it more than likely means you are stressing them out and they ARE NOT going to do it. We may not like it but every 2 yr old knows it's their choice to go potty or not. It has to be their choice to go not yours. Just remember to praise them when they do make the choice to go potty and encourage them.... NEVER EVER discourage them or get mad when they have an accident. It happens we all have done it... it's one of those things we talk about with them and then move on. If you do it's just going to hold them back. You have to stay positive about the whole experience.
3rd... You need to make the choice what is better for you and your child either diapers or pull ups. My personal opinion as a teacher is that a pull up is the same as the diaper the only thing better about a pull up is the child feels more independent because they can pull them up and down themself. When you start potty training them you want to keep in mind that this is something your child is going to want to do on their own so you need to make it as easy as possible on them. you don't want to put them in clothes that they can't do themself. so yes that means no more onsies, preferably no overalls, try not to do any tight fitting clothes (at least not until they get the hang of going potty for awhile). The best clothes you can put them in is a pants or shorts that have an elastic band or something they can pull up and down easy by themself.
Last but surely not least (and maybe even one of the most important parts) you need to stay CONSISTANT!!!!! i can't express that enough. If your not willing to stay consistant and help them with the process it's sort of not really worth trying. The child is going to take MUCH longer to potty train if your not staying consistant with them. They are going to think well it can't be that important to go potty cuz mom doesn't really care if I go or not. You need to find what works for you and your child. Remember each child is different though. Don't push one because the other one is doing better or something. They will get it on their own time.
I usually recomend to my parents in my class to set some kind of timer. (one on the microwave is nice but one you can take with you is much better) Even when you are out you will stay consistant and your child is going to know what it means when the timer goes off. Set the timer for an hour. When the timer goes off remind your child "oh let's try to go potty". When they get done set the timer again. (If you notice they arn't going every hour you can change the time to longer or shorter if they are wet before you try to go again.)
When you do decide they are actually ready for the underwear (staying dry a majority of the time) then you should try them out. Don't freak out about putting a diaper on them when they take a nap or when they are sleeping at night (that's not going to set them back) But DON'T put them in a diaper during the day if they have been in underwear and have been staying dry (that just confuses them). If you go ahead and put them in the underwear and they start having more accidents then they probably arn't ready for the underwear and yes it's ok to put them back in diapers..... it's one of those things you just have to try to use good judgement. Sometimes they need to have the underwear on and have an accident in them to realize they don't like the feeling of being wet. Don't make them sit in them for a long period of time but don't rush the second they have the accident to change them.... they need to realize they don't like that feeling so they need to go potty in the potty.
Oh yeah .... Don't forget to talk to your child about going potty....it's a HUGE milestone/accomplishment for them.
I hope this can help you even just a little bit. Or anyone else who reads it. Sorry it was so long. If you have any more questions though I am here and would be happy to answer them.