auto change cell formula on condition of worksheet change in other cells

Hi Everyone,

I would like a cell to use two formulas (one or other (not simultaneous)),
one primary, and one secondary (if/only when conditions apply).
Can it be done?

ASSUME:
in Worksheet one: C1=A1+B1

in Worksheet two:
A1='Worksheet one'!A1
B1='Worksheet one'!B1
C1='Worksheet one'!C1

----------------------
PROBLEM:
in worksheet two,
ONLY if I change a value for A1 or B1 (in worksheet two),
I want C1 formula to be =A1+B1 (in worksheet two)
otherwise I want it to be ='Worksheet one'!C1

Can I have the formulas tiered?
Can the formula for C1 (worksheet two) change from:
='Worksheet one'!C1
to:
=A1+B1
when and only if A1 or B1 (in worksheet two) change?