I have 21 month old twin girls. We are working slowly towards toilet training and I am beginning to wonder if we will ever get there. But we started just after christmas as they were telling us when they went. They go on the potty when they get up after sleeps, before bed and when they get their nappy changed during the day and if they tell us they want to go. Child care also put them on the potty at every change and when they ask to go. I haven't had any sucess for a while, but I am hopeful. I think its a matter of when you think they are ready rather than a set time.
We are also looking at putting our girls in big beds sometime in the next six months as they still move alot in their cots while they are asleep and are little rat bags at bed time jumping around everywhere, they share a room too.
Good luck with your twins. If you would like to chat my email is email@example.com
Cassie, SA, 19mth twin girls