Then why do you use two fields for this? Could you not just use either a single field or two copies of the same field with one copy formatted with 0, 1 as its value list and the other formatted with 2, 3, 4 as its value list?
Well, the single field is not an option, I would have done that if I could :)
I considered the second one, but my FieldB is actually formatted that way, where the 0,1 is separated out and the 2,3,4 needs to be linked with FieldA.
Yes, but why isn't it an option? If you are only going to allow a value in one of the two fields, it doesn't really make sense to use two fields--which in turn is a slightly more complex design than you really need for your database.
You could use the onObjectModify trigger on both fields to perform scripts that clear the other field.