When you mentioned 2000 amps, it got me thinking how could you approach Zilla performance at Paul's price. Back in the 60's when Detriot built muscle cars, one way they got more fuel through the engine was to add carburetors. Pontiac bolted 2 carbs on their engines and Chrysler had the Tri-power for the Hemi. Since the controller is just a load of mosfets, caps and diodes in parallel, 2 controllers would just be more mosfets, caps and diodes in parallel, but with 2 separate control boards. A bit like having 2 (or 3) carbs on a big V8.
Since you already have a 500 amp controller, you could build a second and test the idea. If it works, then you have 2 working, tested controllers that could be sold when you upgrade to a couple of 1000 amp controllers.
I can't think of a reason why 2 controllers in parallel would be a bad idea, but I don't know if it would work. It would probably need some testing at 12 volts first and see how it goes.
Just a bit of out of box thinking,
Greg