(where "tab 1" and "tab 2" are the names of the worksheets in the same
workbook).
You could also use something like:
=SUM('tab 1:tab 2'!A2)
Note that this formula will include any worksheets from tab 1 to tab 2,
left-to-right, inclusive, so if tab 3 were moved between tab 1 and tab
2, tab 3's A2 would be summed as well.