You could try the following.
1.Assign the speakers you wish to receive the same sound to a master mixer bus (one per stereo pair).
2.Set up a mixer bus group to be used by the perspective for which you want the same sound to go to each speaker.
3.Route this group to send only to an intermediate mixer bus.
4.Set this intermediate bus to send its output to each of the master mixer buses to which you attached the speakers in 1.
5. Assign the output from the perspective to the mixer bus group you set up (in2). Check that the output is only assigned to that output group.
When HW is allocating pipe output for this perspective it will find only one output bus in the group (the intermediate bus) and so will not use an algorithm to spread the sound across the group outputs (as there is only one). The intermediate bus will send the sound it receives on to the master mixer buses that are configured to receive the sound, these in turn send the sound to the speakers.
I think this should work (but I've not tried this particular configuration).
You could include one output pair directly from the intermediate bus (to save on master mixer buses). You could add IR at on the Intermediate bus if you needed to do that.
iain