I'm not sure I completely understand, but.....sounds to me that:
1) the user can enter a value into either A1 or A2, but not both
2) the other is calculated based on the data entered (eg, data in A1,
formula in A2 or vice versa)
3) something else is calculated in A3 based on both A1 and A2

Sounds like a something that could be accomplished with the change
event.

John