Mat, it is not optimal but you can do it. I believe your STR will ping each sub individually and correct it's frequency response. Just hook one sub up to each channel, set up your microphone and go through your room control routine.
Mismatched Dual Subwoofers
Has anyone ever tried integrating dual subs with two different subs that are different sizes? My anthem STR only has Left and Right sub outs so I assume using that will be problematic. Would love to hear other’s experiences with dual subs!