As for weight placement, a good rule of thumb is 60% to the front and 40% to the back. You can certainly get by with a little more or less than that but don't get anywhere close to 50/50 or the trailer will fishtail. When I was picking up a lot of M105s, I'd regularly pull two at a time, one behind the other. When you attached them double, you'd end up with the front trailer being almost balanced, due to the tongue weight of the rear trailer. I'd always bring along 200-300lbs of ballast to add to the front of the front trailer. If you didn't do this, you were in for a heck of a ride. With the ballast added, you could run hwy speeds, and they would trail as well as if hooked single.
If you have a trailer that has a strong enough frame but has too much twist, weld a piece of pipe, down the middle of the trailer, from one end to the other. They have started doing this with higher end gooseneck trailers and you wouldn't believe the difference it makes. Pipe doesn't like to twist and if you make sure the ends are welded in good and strong, it will take most of the twist out of it. I'd say on a trailer like the one in question, you'd want a 4" pipe minimum.