Hi Mark,
First you have to set up 1 or 2 IAC busses in the "Audio-Midi Setup" from the Utilities.
This 2 IAC busses now appear as midi Ins/Outs in Logic. Now go to the Environment Page "Clicks and Ports" in Logic and be sure to connect the IAC bus 1 and bus 2 IN´s (from the Physical IN module) to an e.g. Midi Monitor. Otherwise you will get a midi Loop.
Now you can make a copy of "Vienna Ensemble" to e.g. the Desktop and rename it to e.g. "VE 2". Put it back to the VE folder.
Next you can open VE or/and VE 2 and select the IAC bus 1 as a midi Input on VE and IAC bus 2 for VE 2.
Thats it.
In Logic you now create an "External Midi Instrument" and assigne the out port to one of the IAC busses. You can use all 16 tracks from one IAC bus routed to 16 Instances of VI´s in one VE.
All outs from your Audiohardware (Fireface 800) will be seen by the 2 Standalone VE´s.
Now you can pysically loop back a many outs from the FF outs to the FF ins as you want. If not used for something different I suggest using the ADAT out and just looping is back to the ADAT in of the FF.
Now you can create Inputs or Aux objects in Logic where the Input is assigned to the respective out from the VE´s. (e.g. VE -> FF out 9-10 goes to Logic Input 9-10 ect.)
Best
christian