There is a table posted as State Towing Laws
which includes a column for "two trailers", which might be a useful starting point. The only things I am absolutely sure of are that it doesn't cover the places I know something about (two Canadian provinces), and that it has mistakes in it. What mistakes? I don't know, but none of these summaries are ever completely correct.
For Canada, you could start from RVDA of Canada - Provincial RV Regulations
. I know that one is sort of right for B.C. and Alberta... it just fails to mention a bunch of important restrictions for Alberta. See what I mean about just a starting point?
I would use the table as a starting point, and specifically check each state (and province) that I was seriously considering visiting. Sites such as the FindLaw State Traffic Laws
can be a good way to find the real regulations, which probably every state publishes on an official website.