Otherwise I guess you would have to take a computer on stage and also an audio interface with at least 2 separate stereo outputs?

At that point you’ve spent more money than upgrading to a MONTAGE.

One of the clear differences between MODX and MONTAGE is the digital and analog audio output configuration.
We are often asked about the differences... this is one of the biggest. MONTAGE has 32 bus outputs via USB and an additional analog output.

If you like the MODX... You’ll love the MONTAGE!

Extra Credit:
For those reading this... the MONTAGE has a second pair of Analog Outputs - the Assignable L/R Outputs - if you are playing in a band and as a keyboard player you are asked to be the bass player in addition to your regular keybd role... many find it prudent to route the Bass Part to the Assignable Outputs, and feed that to a separate amplification system... like a bass amp.

This same trick can be used to route all of your B3 Organ Parts to the Assignable L/R which you could connect to a Leslie speaker or Leslie Simulator. The Assignable Outputs are like channel Direct Outputs... the signal routed to AsL/AsR are removed from the System Outputs prior to the System Effects, Master Effects, and Master EQ. The Parts assigned to the Assignable Outputs go to those Outputs with their Insertion Effects intact.