Actually, both the single and dual board versions work the same way; it is just the chips which are a different configuration. In other words, you have six versions:
Area 51, dual board
Maximum Force, dual board
A51/MF Duo, dual board
Area 51, single board
Maximum Force, single board
A51/MF Duo, single board
Someone can correct me if I am wrong on the following. First, the dual board A51 was released. When MF was released, it was released on the newer single board hardware. In order to provide an upgrade path for operators, a chipset was made for the dual boards to let MF play on them (swap out the chips, the PIC and the hard drive). For cost reasons, A51 was converted to run on the single board hardware. Finally, the Duo set was released for both versions of the hardware to allow operators to run both games on the same machine. To summarize, you can get either the single or dual board and run either or both games on it with the right chipset and HD (btw, your 20GB hard drive can be used for either boardset).
Just to be clear I have the individual game chipsets (w/PIC) for both A51 and MF. I would be willing to trade one or both sets for a Gamecube/Xbox/PS2/DS game if you have any to trade. Sorry, can't help you with the holster(s). You might want to try eBay or Happ.