Not to second guess why he wants tandem axles, but why does he want tandem on a small, light trailer?
Tandems do not turn or corner well, they tend to bend the axles as each wheel turns a different radius and fights the other axle for traction. Hard/impossible to manuver by hand also. 2 times as many wheels, brakes, bearings, tires.
Tandem does handle uneven surfaces well since no one tire dipping into a rut/hole will cause the trailer to dip. That's about it for benefit IMO
**add, I had a "13 footer", thats interior length, that had tandem axles. It was on 13 inch wheels. The reasoning, I assume, was ride quality. It did handle uneven surfaces much better than single axle would. The axles did have some bending at the hubs from the stresses common on twins.