At what age would you leave your second child to bath alone with your first child?

[lilima]

I can't leave my DD 15 months, as she tries to climb the side and has slipped.
If anything has to happen to her though, I know DS would yell out immediatly. They were playing on the verrandah this week and I was inside, (but can see them from our kitchen) and DD slipped between the trampoline. DS was HYSTERICAL - screaming for me that she was hurt and needed help. So I know he would let me know, but I would hate for something to happen if they were alone as it would distress DS as well.