Indirect formula

Hello,

The following formula searches the last cell with text in the sheet called '500 Amy' in column A.

=LOOKUP(2,1/('500 Amy'!$A:$A<>""),'500 Amy'!$A:$A)

I would like to replace '500 Amy' with some sort like an INDIRECT formula so that I can reference the cells in column A and B to drag down the column.

For example,

A formula that looks like this but it shows an error.

=LOOKUP(2,1/('INDIRECT(A1&""&B1'!$A:$A<>""),'INDIRECT(A1&""&B1'!$A:$A)

Thank you.

Queenie