You really need to check state by state in the US. Last I looked, 28 states allow towing two trailers, and I have done it for years in the western states, although I'm not sure if I'll add a hitch to the back of my new 5er or not.
Unfortunately, the rules aren't consistent here in the lower 48. Some states only allow two trailers if one is a fifth wheel. It varies on whether or not trailer brakes are required based on weight of trailer, and the total allowable weight varies. Also, if one hitch is a gooseneck it even varies there as to whether or not safety chains are required. While you're checking on trailer laws you can also see if you can right turn on red, see if speed limits are different when you're towing a trailer etc... We don't necessarily make it easy to know what the rules are so that you can follow them.