Is it possible use two or more Midiface 4×4 or Midiface 8×8 at one computer?

No, because the hardware ID of the Midiface 4×4 and Midiface 8×8 is absolut identical and they do not list the additional MIDI ports correct. The computer cannot decide which interface to use. But of course, one Midiface 4×4 can be combined with one Midiface 8×8.