Just told a Buddy my Old method
Live well R/W combo...what I did was put in pump big enough for both. Then on the outlet for the washdown I put a Y (brass Home depot)valve in and used a portable live well tank making up fitting on my own.
Live well tank...find a 25 gallon plastic round tank. Places like car washes etc may have them. Cut the top ALMOST all the way off leaving the lid on to keep closed.
Put a fitting on the bottom that will accept a female hose connector. Take an old piece of hose for feeder to the live well and then punch a hole about 3-4 inches from the top of the tank for an outlet. You could rig a drain fitting up with a hose overboard or to the scuppers.
Placement of pump should be close the bilge pump if you have room in the same compartment if you have the room.
For proper plumbing fittings I went to Ozzies Camping and Camper Sales in Bohemia as they do modifucations and replacement of these sort of fitting used rahter the Rube Goldberg it from Home Depot. Any camper service place will do. They have all the proper gasket also that will conform to the bend in the barrel.
Pump size will vary on size of livewell and r/w washdown if you desire to run both at the same time. 99% of the time with just a flip of the valve you turn one off and the other one on.