"Several freight companies use tandem semi-truck trailers (UPS, FedEX, DHL, etc.) I was nearly certain they were allowed to pass through all states on the freeway in that arrangement."

We see them in NY too. But when they get to the MA border, they have to unhook the second trailer before they cross the border. A second tractor has to come out to drive the extra trailer into MA. There are special lots for this on the interstates right near the borders.

Personally, this seems silly to me (if they're allowed in some states, they should be allowed in all, especially if we're only talking interstates here), but politics is what makes life interesting (to somebody, I guess)...

Mike