The Excel help files explain that:
"You can use 3-D references to refer to cells on other sheets, to define names, and to create formulas by using the following functions: SUM, AVERAGE, AVERAGEA, COUNT, COUNTA, MAX, MAXA, MIN, MINA, PRODUCT, STDEV, STDEVA, STDEVP, STDEVPA, VAR, VARA, VARP, and VARPA."
You can't use 3-D references in array formulas
or with the intersection operator.
If you only have two sheets that require the SUMIF,
you could use
=SUMIF(Sheet1!A:A, B1, Sheet1!A:A)+SUMIF(Sheet2!A:A, B1, Sheet2!A:A)
or, you could set up the cells in column C on each sheet so that they looked like:
C1: =IF(A1>2000,B1,0)
You can hide column C if you need to.
then you could use
=SUM(Sheet1:Sheet2!C:C)
Hope that helped.
, A and B