Check states laws. Lots of variations. From what I've found, most western states allow it except OR and WA, but once you get east of the mountain states, then its a no-go.
Most trailer MFGs do not allow putting a hitch on the back. Northwood's chassis allow it, and even offer it as a factory option. I would not trust most Keystone frame for double towing. Their chassis seem often seem marginal as-is.
My trailer still pulls great with a 2nd trailer, however I'm 75' overall and have gotten myself in places the whole combo would not fit, and had to back up nearly a quarter mile. Backing doubles isn't exactly fun. I have an array of LED backing lights on my TT, which is absolutely necessary when backing doubles in the dark.