DH decided when on paternity leave for DC2 to give potty/toilet training for DC1 another whirl. We have been trying for a year with some success, but some major setbacks related to "accidents" and hysterical reactions from others when not in our care. 
The good news is that DC1 is now doing brilliantly, almost no accidents a month on. Even if I did want to kill DH at first!
The thing I'm wondering is if she's using it as a way to get our attention away from DC2. DC1 invariably needs to go to the toilet just as DC2 is screaming for a feed (am bfing) or just as he's started a feed, which results in me having to put him down to help wipe her bottom and make sure she washes her hands. And he screams.
She also goes a LOT.
I've tried letting her go alone, but she can't yet do the wiping herself and skips hand washing given the option (then the next thing she does is try to touch DC2).