Assuming Bale twin #1 is the bad-ish one who ties the fatal knot and twin #2 is the meek and gentler one and that Angier met twin #2 both the times when he wanted to know which knot had been tied, why didn't twin #2 lie to Angier?
It's not as if he has sworn to tell the truth throughout his life. Why not tell him he tied the simple knot instead of giving him an annoying and obviously impossible 'I don't know'.