The method I used and swear by may be hard to do with two kids at once but I'll describe it anyway... We went straight to underwear all day long at 19 months and had a lot of accidents for a few days. We just kept a potty close and I moved her there every time she started to go, rewarding her any time even a drop made it in, and reminding her that's where she should put her pee and poo. She was trained quickly. You really have to watch them closely and plan to do nothing else for 3-4 days. What happens is in the first day or so, they learn to hold their bladder if they weren't doing this before, so that they empty their bladders when they're full, instead of a little bit every few minutes. Then after that they start to learn the "need to go feeling" and associate it with the potty. You just keep working with them until they learn to tell you. It takes away the power struggle of making them sit and wait on the potty.
at 11:56 AM on Jan. 8, 2010