Are the Main and Assignable outputs identical if nothing is assigned to the Assignable L/R?

No. They are quite different. The Main L&R include the SYSTEM EFFECTS (Reverb, Variation), the Master Effect, the Master EQ. The Assignable L&R, is literally, a pair of analog outputs that when you assign a Part to them removes that instrument from the main (system) outputs. It is done so you can isolate that Part for separate processing. If you have a situation where you want to separate a Part or Parts of your mix of instruments you can isolate that Part by routing it to an Assignable Output (there are some 30 assignable USB Outputs, but just 2 analog).

If nothing is assigned (routed) to the Assignable Outputs they are silent.

Or can it be assigned the same output as the Main outs?

How you use the Assignable Outputs can vary. But the short answer is, No if you mean just routing something using the Montage itself. I explained above, assigning from within the Montage would isolate the assigned Part, separating it (removing it) from the rest of the Parts that are feeding the System Output (MainL&R).

That said, when using the Montage in a computer based setup with a professional DAW, like Cubase Pro 8, you can retask the "Assignable L&R" outputs. It can be used to create a completely separate mix... Which can be fed to another pair of speakers or run a headphone amplifier system.

In a situation where you are recording with the Montage via its 32 USB output, 6-audio return system, you can use the first Return to feed the Main L&R jacks for your main mix, but return audio from the DAW directing a second mix to the Assignable Outputs. This could be an entirely separate custom mix, for example, a typical use would be to create a mix for the recording musicians via headphones.

Cubase Pro allows you to setup several different bus routings. And the Montage features 3 pair of audio Returns:
Main L&R - feeds the Main L&R outputs
USB L&R - feeds the internal mixer
AsL&R - feeds the Assignable L&R outputs

What are you trying to do? There is no reason to duplicate the Main L&R at the Assignable L&R, other ways exist.

Thanks BM. I understand your explanation. If they were similar outputs I thought it would be an easy way to feed my monitors and a video camera or two different pairs of monitors without using an external mixer.

Nope, good reason for a small format, high quality mixer...
Yamaha AG series