That's the correct syntax, but you need to pay attention to the relationships linking your tables 1 and 2 to the table to the record where you define and evaluate the sum function.
Sum ( Table2::Field ) will total up all the values in Field for all related records in table 2 and that may not be what you expect here.
Sorry, I ment to include the example as follows:
All of the fields are in the same table.
I get the correct sum if I focus on one field, but when I add the second field the results are inaccurate.
But what is the context of the Sum function?
Is this defined in a different table than Table1 or the same table?
If it's defined in a different table, then what I said in my previous post still applies. You'll note that in my last post, I only had one parameter: Sum ( Table1::Field1 ).
Likewise, if you put this in a script step, and the current layout refers to a different table, the same is true.
By the way, if you just want to add two fields, you can simply use Table1::field1 + Table1::field2