Trigger grants

Here is my scenario:

USER1 - owns TABLE1

USER2 - owns TABLE2

I created a trigger that will insert a record into TABLE2 when a record is inserted in TABLE1. 

grant insert on USER2.TABLE2 to USER1;

when I try to enter a record in TABLE1, I get an error:

ORACLE error:  unable to insert record

What access needs to be granted so TABLE2 can have a record inserted when a record is inserted in TABLE1?

Thanks!