From the owner’s manual:
"GAIN 1 KNOB (OD1 & OD2 ONLY): Controls the amount of gain in the preamp section.
GAIN 2 KNOB (OD1 & OD2 ONLY): Sets your 2nd gain for OD1/OD2 channels with the dedicated 3-way BRIGHT switch. "
I’m fairly certain that in the real amp Gain I feeds into Gain II, so it’s safe to asume one affects the other.
Might be worth checking the owners’ manual of the real deal: https://www.fortinamps.com/wp-content/uploads/2019/05/CALI_OwnersManual.pdf