Could you not have two relationships, one based on FieldA = FieldB, and the second on FieldC = Field D, and them in the calculations, have them prefixed with a case Statement:
FieldA = FieldB OR FieldC = FieldD ; DoThis, etc
Maybe I do not understand, but a multi-predicate relationship should be a solution for you.
You'll need to set a field with two values from one side of the relationship to get the OR.
I played with that idea before I posted, and I couldn't get my head around whether that provided the right combination of AND and OR - but I am sure raybaudi is right!
raybaudi's point is that if you have a field on one side of the relationship that has the return-separated values that in one record has these values
and another record with the values
and a field on the other side of the relationship with the values
both will match for a OR c.
Thanks everyone! Sure enough, return-separated values in a match field do the trick!