The Valtera drain valves are different than the Drainmaster. You cannot just add a second switch. However, I did add switches to my Valterra valves, as well as remote switches for the tank fill valve, black tank flush, macerator and even Blue Power chemical tank pump to add chemical to the black tank. Those extras simply required switches but in order to get the dump valves to work remotely I had to use a pair of relays on each dump valve to make that happen.
I wanted to stay with the Valterra valves because they use steel gears rather than nylon as used in the Drainmaster. The problem is that the valve gets power from the rocker switch and once the valve moves to its location the circuit is interrupted so that there is no more power draw. The A and B wire connections reverse polarity and when the switch is moved the second time the power is reversed and the A & B becomes B & A. This works well but you can't simply add a second switch without getting things all crossed up.
By adding a pair of relays you can allow a pair of switches to control the pilot relay while the secondary relay controls the polarity. It takes a bit of wiring but it's not hard to do. The hard part was figuring out the wiring schematic. The nice thing is that I can dump the holding tanks, fill the fresh tank, flush the black tank, add chemical to the black tank and even operate the macerator (if I'm using it rather than the 3" slinky) all from the mid-bath while watching out the window as the rain is falling.
Attached are two of the schematics I drew up. Note that I added a pair of 12 volt solenoid operated water valves for the black tank flush and also for the fresh tank fill that I ordered from Amazon. These were connected parallel to the manual valves in the basement. The chemical tank is actually the same as the windshield washer fluid tank and pump used by Entegra. It's the same as a Bluebird bus and I believe you can get them from the same place (Wiper Technologies) you can get windshield wipers from. The rocker switches are standard mini-rockers with pilot lights that illuminate when the switch is in the "on" position. The relays are DPDT with 12 volt DC coils that I got from Grainger.