You don't need to ask somebody else. The input is documented as mono and behaves like a mono/stereo summing input. If you don't trust the documentation the test is trivial, feed it a stereo test signal and pan the signal hard left and hard right. In this case the stereo image will not pan on the outputs because it is being summed on the input. To be most convincing make the stereo test input L/R tracks identical so there is no signal change, just a level shift.
The most relevant discussion I can remember on DUC was where Chris Townsend commented/implied that there was probably enough DSP processing power in the Eleven Rack to allow running two amp sims... if Avid ever released the software to do that.
Jeez this is even question #1 in the Avid Eleven Rack FAQ
http://avid.force.com/pkb/articles/en_US/FAQ/en359231
Darryl