No. The Assignable Outputs are designed so that you can remove any Part (or Parts) from the SYSTEM routing and isolate it on its own Output. Since any Part, or Parts, you route to the Assignable L&R would go to that Output with their own Dual Insertion Effects but before the Sends to the System Reverb and Variation Effects and before the Master Effect and Master EQ. You would use the Assignable Outputs in a case where say you wanted to run your organ sounds through a Leslie cabinet, you can store your organ PARTS so that they go directly (and discreetly) to the Output not to the Main Outputs. If you want to use an external effect processor like a NeoVentilator, you would route the Organ PART OUTPUT to the Assignable L&R. Anything assigned to the Assignable L&R is removed from the normal path to the Outputs.
Other uses: If you want to isolate your vocal mic, plugged to the A/D IN, to its own speaker, or if you want to take your MONTAGE lead guitar sound and route it to an actual Guitar Amp or process it externally.
If all you wish to do is duplicate the Main L&R, typically what is done is -- get a device called a STEREO SPLITTER. It takes in L&R signals and gives you a duplicate output (usually balanced). A Stereo Splitter would take in the Left and Right analog signal and give two stereo signals. It basically splits the single (with no loss).
Or you could get a small format mixer to do the job.
... it's not that Assign L&R couldn't duplicate what would otherwise present on Main L&R exactly. Main L&R can turn off effects - so you could turn off the same effects for Main L&R so it would match what would output from Assign L&R. Or, instead of turning off any effects - if your Performance happens to have no System/Master effects - then this could also have a twin between Main and Assign.
Yes, the "normal" day-to-day Performance will have more effects enabled on Main L&R than Assign L&R can send - but there are certain cases where the two will be "identical".