this is a old argument.
towing doubles is legal in all 50 states. pending.
first:
one must have a class A driving permit.(some states require it to also be a commercial class A, with doubles endorsement)
second:
the tow must be a pickup truck, (most states require them to be registered as commercial). yes you can register as another,(show, rv) but you must jump through a bunch of hoops to get and keep that way.
third:
MUST not exceed the trucks gvwr. (so must be at a min a 3/4 ton. better 1 ton-up).
forth:
can not be longer than (65 feet--bumper to bumper-- in ca). your state?
fifth:
most states require the first trailer to be a fifth wheel.
sixth:
got proper insurance?
seventh:
some states require the second trailer to have active, not just surge brakes.
eighth:??
now this is for the USA, sorry can not say for canada.
there are some roads and minor hwy's with size, width and length restrictions across this country. some due to bad, or reduced, clearances, or rd conditions or weather can prohibit double towing.
but on all interstate and most state hwy's it is legal to double tow pending the above.
this is from what i have read on some states laws,(not read up on every state, to many laws to search out here), talking to fed and state dot officers and dot inspectors.
i know personally no problem in these western states. Ca, Or, Wa, Nv, Az, as i have not had any problems, from state, fed, local, or tribal police agency's. been too many miles to not have been seen by all of them.