If there is a way to dual mod non common ground pads, I have not seen it.
Your best bet for a dual mod would be a Cthulhu and a MadCatz 4716 pad. You can find both at gamingnow.net, but I got my MadCatz pad from the local Gamestop for $25, and I didn’t have to wait for shipping.
Use this guide to get your MadCatz pad working.
From there, you have a few options for connecting the two PCBs together.
This guide lays it out for you.
I haven’t personally worked on a Mayflash stick, though my experience with others would say that the only other snag I could think of would be in terms of the four buttons at the top of the box. Generally since you can’t just replace them with some good Sanwa’s or Seimitsu’s due to size constraints, it’s just recommended that you disconnect the small PCB under them from the PCB that connects to the 8 face buttons, and from there it’s up to you on how to connect them to the Cthulhu or MadCatz. Some guides show people soldering directly onto the contact points, but that often causes the buttons to become stiffer due to the added wire. Personally I find it much easier to solder onto one of the traces instead, but that might end up being more difficult depending on the manufacturer.
Sorry I can’t provide instructions specific to the Mayflash stick. Hopefully they haven’t changed too much of the design to make these instructions useless.