I have double towed with my previous TT, a 22' TT and a 14' Landscape trailer was the longest (length and miles). I was 60' Bumper to Bumper and towed 600 miles round trip to the salt flats and back once.
The 2nd trailer has a greater propensity to sway, but I didn't experience anything dangerous or unstable at speeds up to 65.
Most of my double towing was local with a jet-ski or 4wheeler, or both, behind the TT for 100 miles one way at most.
I have a custom 2" receiver on my current TT, just haven't had to use it for a trailer yet.