If you are using a SAS data step, you can't reference a varibale by dataset, like you would in i.e. SQL. (I guess you are doing a merge)
data table3(drop=Table1_Var Table2_Var);
NewVar = sum(Table1_Var,Table2_Var);
The rename will not change table1 and table2, the rename will only work in the current datastep.
Using "+" instead of "SUM()" can/will give wrong results if you have missing values - I changed that in my example.