Dyno Test Rig

I'd probably run another instance of VESC tool and monitor the second controller there?

What would you want to command to the generating motor/controller? You can just have it in a constant regen mode with a desired amount of braking amps configured. 

Gotta be careful how you connect the power supply, battery, usb etc. to ensure you don't have ground loops. Those will kill controllers in an instance. Also, depending on what kind of battery you use and its state of charge, the PSU may not reverse current on the battery and potentially all the way back to PSU. I would just run it all from a battery and disconnect the PSU during testing.

Thinking a bit more about this, I wonder if using an ODrive would be a good fit for a dyno.  They come with support for braking resistors.  The default is pretty small but I could easily solder on something that could soak up a couple kw, so you could power it with a bench supply and possibly not have to worry about blowing it up.   Gahh, now I have another thing on my todo list.

Edit:  Interesting discussion here:  https://discourse.odriverobotics.com/t/motor-efficiency-dyno-odrive-cont...

If the goal is to create the load, simply attach a spool of thick wire to the phases and put it in a bucket of water. That's effectively your resistors with ability to absorb many kilowatts of braking power.