The firmware that it ships with is for a different model so it doesn’t work quite as well. Here is firmware that’s configured specifically for that model:
If you want more speed, running firmware 5.3 or higher can take advantage of field weakening which will increase your top speed but will significantly reduce range.
I’d leave the two batteries in parallel if they are the same voltage but different capacity. I run a 12ah battery with a 38ah battery in parallel. If you put them in series, you would need some sort of low voltage cut-off or alarm for the smaller capacity pack since it will drain faster. One of these cell beepers or a 20s 100A+ BMS (or 2x 10s 100A BMSs) where the discharge is wired are a few ways to detect when the smaller pack is empty to prevent over-discharge.
Running 84v vs 42v would pretty much double your top speed, and the controller can handle it.
Motors can run at pretty much any voltage. The limit is usually bearing or high iron losses (aka excessive heat) at high enough RPMs, but neither of those things should be a factor in a relatively low RPM hub motor.