There are many ways to go about this. To list a few:
X-arcade + 2 PC gamepads. Leave X-arcade alone, but remap mame so the gamepads control players 3 & 4 instead of 1 & 2.
X-arcade + u360/gp-wiz/ or gamepad hack. Same as above, but make a CP for p3 & p4.
X-arcade + ipac2/key-wiz. Again, leave X-Arcade alone, but remap the ipac/keywiz encoder. Remap mame to match the encoder.
I didn't know the x-arcade could be remapped. If it can, what Necro said.
X-Arcade + ipac4 + parallel wiring the x-arcade to also the ipac4. Ipac4 hooked to PC, x-arcade hooked only to dreamcast. Careful about one system shorting the other. (IMO hard.)