Here are a couple modified Sheridan cartridge valves.
The first would be very simple to achieve with a standard PGP2k valve. Note the front left valve, with slots milled into the sides. The front and rear orings would have to be in good shape to hold air, which isn't really the case on the stock gun.
The payoff is a huge valve volume, which would make getting double barrel guns up to high velocity an easy feat.
The second valve has separated the valve body and check valve into separate components. In a PGP2k, the check valve would be replaced by the chamber plug and 12 gram seal.