All of this advice is good so far, I just want to condense a few things for you. First, make sure you see all three signs of readiness from both twins (and who says you have to potty train both? If one is ready, start with her and I can almost guarantee the other will either become ready right away, or when she is ready it will be a breeze) Here are the signs
- She is physically ready (can "hold it," can pull down a pair of sweat pants and underwear)
- She is cognitively ready (can understand the concept, can tell you when she needs to go)
- She is emotionally ready (basically means she wants to)
If one of these areas is lacking you will have much more of a struggle on your hands. Ignore age (kind of)- some girls are ready early, some are not.
I agree to avoid pull ups, except maybe at night if you don't want to use diapers still at night.
Rewards are up to you. There are reasons they help, and there are reasons to avoid them. If you do use them, you could run into problems when you stop giving them a reward. My suggestion would be to start without, then use rewards possibly if you need them (although if you need them, it may be a sign they're not ready). Or the reward could be something like if they have two days with no accidents (just during the day) they get to pick out some really cool underwear. This would be appropriate because it is related to the desired behavior.
Lastly, when you are ready to potty train at night, too, you're probably going to have to wake them up at least once every night to get them in the habit of getting up to go potty.
Most of all, do what works, and like the first mom did, if it's not working, scrap it and try again in a month or two.
Good Luck!