Picky, congratulations!! What incredibly wonderful news. You must be over the moon and to have twins too... wow! Sounds like you found some wonderful donors - just wonderful.
JFB, I can remember A at about 18-21mths showing great interest in the toilet and wanting to sit on it regularly and I am now kicking myself as I should have jumped on it and used that enthusiasm to TT her. I have read that around the 21mo mark is a great time to TT if you can, so you might be surprised if you keep it up, she may TT very early.