You don't. You're trying WAY too hard. Plus, potty training is the one thing that you have absolutely no control over. You can encourage it sure, but it isn't like having them clean up their room or finish their dinner or something...with this, the bodily functions are totally up to them. And many times, the harder you push, the more they'll resist, and you've got double-trouble with trying to get it done with twins!!!
I'd wait until they are closer to 3 to 3 1/2 first of all. BUT make sure you have potty chairs for both of them. You can introduce some sort of reward system like a chart where you put stars on it every time they potty in the potty. Or you can set the timer every 30 minutes and have them go sit on the potty whether they have to go or not, or they just went.
What we found worked best was whenever they wanted to switch activities or do something different, get a drink or have a snack, watch a show, whatever. Every time..."You can have a snack, but you have to go potty first." "You can watch your show, but you have to go potty first." "You can color, but you have to go potty first." And every time before you go to leave the house for an errand - everybody goes potty.
For what it's worth....and good luck!