Calculating across tables

Hello, Newbie question.
I have 2 queries from two datasources. I can merge these 2 queries and display the result like this
Cat, Value from T1 Value from T2

Cat1     100          110

Cat 2    100          100

Cat 3    120          100

I want to add column Delta which should be Value fromT2 - Value from T1

Then filter on that to retain Delta <> 0

Seems simple but I cannot get it to work.