Well, the question leads to another question? What are you attempting to do? If you are attempting to do a Surround Sound configuration, the answer will be found in the manual of your Surround Sound capable mixer. If you are just attempting to connect more speakers, the question would be, Why?

Three or four speakers without an intelligent distribution system does not get you anything but perhaps in trouble with cancellation and phase problems. The 'intelligent distribution system' would be a mixer that allowed incoming signals to panned around in what is technically called "Surround Sound".

Digital mixers like the renowned Yamaha 02R96 have full Surround capability complete with joystick control.
You can/should read up on Surround Sound... because just connecting additional speakers doesn't do anything special for you particularly if they are all aimed at the monitoring position. If you need to place a second stereo mix in another room (for example) connecting a second pair (left/right) could make sense. The wiring would depend on whether the speakers are powered or you're using multiple amps.

The other use would be having two sets of Monitor speakers... a large pair and a small bookshelf sized pair, to make A/B comparisons. Back in the day, recording studios would have large Urei or Altec Big Red speakers mounted in the wall playing toward the engineer's monitoring position, and a pair of bookshelf sized NS10M or Auratones as the small near-field monitors.

You can, when using the Montage as your audio interface, setup an advanced DAW, like Cubase Pro, so that the first stereo Out goes to one pair (Main L/R) and the second Stereo Out is fed from the Assignable L/R... you can play through one *or* the other in A/B comparison fashion.

Because the Montage has three pair of audio returns, two that can feed external speakers, you can return two different mixes to the Montage outputs... one could be for the engineer (critical listening) the other could feed a monitoring system for overdubbing musicians (subjective listening). You can create two entirely different mixes for each set.

See Cubase Pro documentation for setting up such a configuration.

OK, fair enough, I guess I need to explain what I am trying to do 🙂

I am trying to create the "illusion" that when I play the various grand pianos (CFX, Bosendorfer, etc.) on the Montage - that the actual CFX/Bosendorfer is right in the room (room size 14' x 20').

And also to give the illusion that the piano can be in an actual cathedral, concert hall, etc. if I choose.

I am not sure I can get that with just the two, left and right, monitor speakers...?

I may be able to get that "illusion" at my sitting position - but no one else in the room really would get the feeling that there was a real CFX/Bosendorfer in the room ... no...?

So the only real way I know how to do this would be with multiple speakers, correct?

Hello G - I don't think so. If you did have a real Bosendorfer in your room, the sound would emanate from it - one place. Human ears are accustomed to this arrangement, and to the resulting natural reflections and decays. Another sound source would be confusing. I mentioned earlier that a large room like yours would benefit from larger speakers (I used 12" Torques), set behind you and about 3 metres apart, aimed at a point about 2/3 of the way down the room, where I imagine your audience would be sitting. The room would filled with sound, believe me, and only the one set of reflections to contend with - far more realistic than another set popping up!

So the only real way I know how to do this would be with multiple speakers, correct?

Two speakers is enough to create this illusion. Stereo is illusion. The fact that when sitting in the "sweet spot" you can pan something so it sounds as if it moves across in front of you, is an illusion. When pan instruments in a mix to represent a band in front of you, you using that illusion.

Four speakers in a Surround configuration would give you a broader illusion... ususally what is sent to the "rear" speakers is the reflected ambience that gives and improves the illusion you are in a specific sized room. Where now sound can extend not just left and right, but front to back. But you need an intelligent mixing device to accomplish true Surround panning.

Before you go setting up four speakers, exhaust two (it's been standard for many years now). If you have good accurate monitors, you should be able to "feel" the stereo when you play the "CFX Concert" or the "Imperial Grand" - you should be able to hear the positioning of the string sections when you play "Seattle Sections" as they are spread out left to right: 1st violins, 2nd violins, violas, cellos, contra basses...

I would recommend researching studio setups or talking to your local professional. Your thoughts are not conventional and would be confusing to any listener. This is not a THX movie theater with bangs and booms and gun fire going off behind you. You would get an artificial recreation with monitors competing with one another. Conventionally you want as few drivers as possible all in phase to recreate a realistic reproduction. That's why almost all monitors only have (2) drivers per L/R channels max and not 3, 4, or 8 like some crazy home stereo systems. The more drivers you activate the more problems exponentially you'll introduce into the equation. Then again if "realistic" is not your goal then you can add a thousand speakers and go crazy. You'll need a audio system that provides all the channels that you desire. Here's an analogy. You have a rifle barrel with one scope to aim at your target (conventional studio monitoring system). You want to put 3 scopes on the same barrel (Multi-monitor groups). The first to look through the second and the second to look through the 3rd and then the 3rd to see your target. That gets impossible as you can imagine. Or instead of looking through one window to see outside clearly you're looking through many windows in a series which begin to refract, reflect and obscure one another.