To add to Phil's point, even private individuals will tend to link the FW and RW for a journey. Therefore, unless the aircraft are positioned by someone else, you will only be useful for one of the legs of the journey.
From what I have seen, you will have a primary role, and will occasionally get to fly the other. There will be little savings for the owner as he will not really be able to hire less people, but it may make life easier for the rest of the air department in covering rosters over vacations and due to sickness.
Increase in salary - probably not. Funding and currency to maintain both licenses - possibly. That's not a bad deal.
FFF