Kenny
I have a GC2310 and plan on purchasing your ROPs Brackets.
Is it acceptable to mount 2 lights on each bracket?
Sure, look at some of the pics on the website and the intro post I linked, some have even mounted 3.
Do the mounting bolts that come with light kits fit the bracket,
Yes, they are M8-1.25 thread, but they are probably to long, and since they have a hex (10mm) head they will not turn in the bracket they will be difficult to use.
or do you advise purchasing additional mounting bolts from you?
Well as stated they come with 2 each already so they may be enough for your project.
Below are the lights I am considering.
Nilight 2PCS 18W 1260lm Spot Driving Fog Light Off Road Led Lights Bar Mounting Bracket for SUV Boat 4" Jeep Lamp
https://www.amazon.com/dp/B00G620ZVU/ref=cm_sw_r_cp_tai_NeOODb1MGRYMR
Those are fine, pretty much what we used on the underside of our display and in the one picture on the website.