Yes, you can use up to three RME Fireface800's together on a single computer, Mac or PC. However, you have to have a firewire 800 card in order to do this. Firewire400 does not have the bandwith to handle multiple Firefaces.

The multiple Firefaces must also be syncronized together via word clock or ADAT. Once installed, the Total Mix software will have a different tab for each Fireface. The audio software will also show the inputs according to their respective interface. For Example Sonar or Cubase will show Fireface1-input1or Fireface2-input4, etc...

Check the RME website for qualified Firewire 800 cards that they have tested their interfaces with.
www.rme-audio.de