Yes, the IPAC is essentially a keyboard emulator. You can use two at a time if you use the USB option, or one PS/2 and one USB. The software is used to program each IPAC individually and the settings are written to the IPAC. So IPAC #1 would control players 1 & 2 and IPAC #2 would cover players 3 & 4. MAME wouldn't need any special configuration, just program the IPACs with the default MAME keys. They will both need custom mapping, but it is easy with the IPAC software.
Personally, I would go with the IPAC4 right from the start, just to keep things simple and not burn an extra USB port.