if the two tables are exactly the same in structure, just different in contents, you could do it like this:
first make a Union query to add the two tables together; like this:
SELECT Table1.* FROM Table1
UNION SELECT Table2.* FROM Table2;
Save this query as 'BothTables' or something
then you can use DCount in the conditions column of your macro, like this (presumably your search ID is coming from a text box on a form):
If the field in the table is a number type field then:
DCount("[MyFieldNameInTheTable]","[BothTables]","[MyFieldNameInTheTable] = " & Forms![MyForm]![MyTextBox]) = 0
if it's a text field
DCount("[MyFieldNameInTheTable]","[BothTables]","[MyFieldNameInTheTable] like '" & Forms![MyForm]![MyTextBox] & "'") = 0
But Pdx is right, VBA is easier for this kind of thing.
Mike