I'm about to install a Protools rig and the one box I can never remember the correct cabling for is the XMon - so here is a list of the i/o and what you need to do;
CUE inputs: These are the feeds you are going to switch through to the headphone/monitors (typically the booth in a TV suite) - these should be normalled to the outputs of the 2nd 192 interface.
MAIN inputs: These are what will be fed to your surround speakers and so should ideally be normalled to the output of the first 192 interface.
SURROUND inputs: These would typically be the confidence return from a Dolby encode-decode path - so you could monitor how the surround mix is going to sound once it's been converted.
STEREO inputs: Like the 2TR return on an audio mixer this is the switched monitoring return of the VTR (typically) - folks often put the unbalanced output of the computer and maybe an iPod jack on i/p's 5-8.
CUE outputs: These will feed the booth headphone amp and in the case of a music studio any floor wedges.
MAIN outputs: These feed your 5.1 (or 7.1) monitoring system, your expensive Dynaudio speakers (for example).
ALT outputs: Used for grot monitoring - typically a stereo set of speakers (small ones) to test for domestic compatibility - can also drive a 5.1 amp.
TB/LB/Util: This D-type has in and out - an extra T/B mic for example, and the feed to the big plasma TV's speakers.
So there you go - remember that all eight of the D-types can carry eight audios, but in the case of none music studios (film & TV) you're unlikely to have more than stereo cue monitoring (for example).