Since the highest possible capacity of one axle is 3500 pounds, barring some kind of wacky 8 lugged monster,
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No disrespect intended but your statement above is incorrect. I have a couple of single axle trailers here on the farm with 5000 axle capacities.
Typically you can determine the rated axle capacity by counting the number of lug nuts/studs on each wheel. 5 lug axles are typically rated at 3,500 lbs, 6 lug axles are typically rated at 5,000 lbs and 8 lug axles are typically either 7000 or 7,500.
The real issue that the OP faces is not towing capacity, but braking capacity. Many 3,500 lb axles do not have brakes and depend upon the braking ability of the tow vehicle. In reduced traction conditions such as rain wet roads, that TACO will be very marginal with respect to safely stopping the combined load.