You can use this formula, which is assuming that the data will be in
column A:A in both sheets 1 and 2 (assuming sheets are called Sheet1
and Sheet2)

=IF(ROW()COUNTA(Sheet1!A:A)+COUNTA(Sheet2!A;A),"" ,
IF(ROW()<=COUNTA(Sheet1!A:A),
OFFSET(Sheet1!$A$1,ROW()-1,0),OFFSET(Sheet2!$A$1,ROW()-COUNTA(Sheet1!A:A)-1,0)))

Start this formula in A1 of Sheet3 and copy down until you get empty
cells.

Does this help?
Kostis Vezerides