I towed a 10' box trailer behind my 5er for over 10 years with never a problem. The trailer contained either a quad, golf cart used for handicapped wife, or her handicap scooter, along with a generator and gas cans, etc. That was mostly in Texas, boondocking while attending races at Texas Motor Speedway. But we sometimes ventured into Louisiana, Arkansas, Oklahoma and New Mexico with that rig. We tow the 5er at 62 MPH, so over the years numerous State Troopers passed us and never gave us a second look.
1] Be sure your receiver on the back of the 5er is strong enough for the weight of the trailer. Any good welding shop can add junk iron to the trailer frame area to beef it up enough to install a class III receiver that won't fail.
2] When the 5er does a jig, the second trailer will do a huge jog. Have someone else drive the rig on a crooked bumpy road while you follow in another vehicle. You'll probably be amazed at how much jogging the second trailer does. And you cannot see the second trailer from the tow vehicle, even with towing mirrors, so you need at least one sway bar on the hitch for the second trailer so that huge jog doesn't become uncontrollable sway. I used an EAZ-Lift sway bar like the one in this photo from the EAZ-Lift website:
That photo also shows the EAS-Lift weight-distributing hitch, but we used the sway bar with an ordinary weight-carrying ball mount, without the WD hitch. Worked fine to control sway.