You can't have anything other than a single field on the left side of the =.
I don't think you can do what you want with set analysis unless either Field1 or Field2 has just a single possible value. A set is evaluated once for the entire chart, so if both fields have multiple values, you can't use set analysis for the chart. If Field2 has a single value, I believe you could write it like this, and something similar if Field1 has a single value:
If Field1 and Field2 can have multiple values, you'll probably need to do it with an if(), such as sum(if(Field1-Field2<29,Value)).