I want to create a formula with variable sheet names based on the contents of
a cell.

A1 = Sheet1
A2 = Sheet2
B1 = yes
B2 = No

=IF(B1='yes','A1'!X1)+IF(B2='yes','A2'!X1)
Where A1 and A2 resolve for the contents of A1 and A2 resulting in:
=IF(B1='yes','Sheet1'!X1)+IF(B2='yes','Sheet2'!X1)

Any help would be greatly appreciated.
Mike