No, that isn't how that works at all. NA flights are priced one way so any change can be made to just one direction without impacting the other one. I can't count the number of times I've changed one direction without affecting the other.

The only potential link between the two IIRC is that the entire ticket is subject to the cancellation fees of the lowest fare class (i.e. Can't refund the Y portion of a T+ outbound, Full-Y inbound ticket).