Here in the southeast, its not allowed in the vast majority of states. That being said, I too am seeing it more and more. Last week at Ocean Lakes in Myrtle Beach, I witnessed several tandem set-ups. Ocean Lakes is a HUGE golf cart populated campground. Over 800 sites and at least half have golf carts I imagine.
I saw multiple campers with trucks, the camper, and the utility trailer for a golf cart. One day, I saw a guy leaving with his truck, his travel trailer, and his utility trailer with a golf cart. Rarely see the travel trailers pulling another trailer but it seemed to be pulling just fine.
As has been mentioned, I have never seen one pulled over or heard of such by the law.