I have experience with Kodiak single axle disc brakes on my tandem axle trailer.
21'WAC, 150 Optimax,aluminum trailer I figured 4500LBS fully loaded with fuel water and camping gear,being pulled by a 5,000 rated Isuzu Trooper that weighs in at 4,500 Lbs.
I have them on the front axle only,and never had a problem stopping.....even stops better than just the Trooper with no boat/trailer behind.
I changed over to Tie Down engineering brakes cause they were cheaper to replace the whole set-up than Kodiak (long story,don't let them sell you lifetime replacement car brake pads cause its the same pad
The Tie Downs suck!!!I've thrown the center caps off on a 400 mile trip,and they don't stop as well.
I will be going back to the Kodiaks,but will just stick with one axle at this weight.
Also,in a disc brake set-up,you can not back up unless you have a lock out solenoid or a pin to de-activate the trailer brakes,so you have no brakes when backing down a ramp.If you haven't done so,I suggest the solenoid hooked into your brake light circuit to lock out the brakes in reverse instead of just pinning.They sell a different brake actuator/coupler for disc brakes that has this.
There were times in maneuvering that I needed to get out and throw in the pin,and this could be bad if you need to back up suddenly to avoid something,like someone backing out in front of you!!
So my advice would be to install on one axle,try it and see if you think you need the second axle....it's not much to hook the second one up if needed,just a few extra fittings and hoses.
PS,from a professional driver...if your brakes are locked up and smoking,you have less braking than when the wheels are still rolling with brakes on....this has been demonstrated to me in my extensive training and re-training over the years.