Great, another freakin' oil thread.
I've given up on rail trailers for bikes. Too many slips and falls. I'll take a solid bed every time. Virtually every problem I've ever seen anyone have with trailering motorcycles stems from cutting corners on equipment and/or neglect. You don't have to pay extra for Harley or Beemer branded gear, but it is not wise to cut corners too sharply.
Anywho, I've been quite satisfied with my Harbor Freight cheapo 4x8 kit. Best bang for the buck. Be sure to spring for the 5.30-12 LRC 6-ply rated wheels and tires. The bigger, heavier-duty wheels and tires provide a smoother ride and last longer than smaller tires, and the difference in cost is minimal. Be aware that trailer tires have relatively thin sidewalls and will normally lose pressure over time. Be sure to max out the tire pressure EVERY time you hook up the trailer or the tires will fail from under-inflation or overloading that cause heat to build up.
Clean and repack the wheel bearings and hubs with a real quality wheel bearing grease. The provided grease is no better than slime scraped off dead fish. It actually smells like fish slime. Repack wheelbearings every 2 years or 10,000 miles and you'll never have a problem.
Take the quadralateral piece that goes on top of the junction of the tongue and the two angle brackets and duplicate it in 1/4-inch steel. Bolt the extra reinforcement on the bottom of the tongue and brackets directly below the original.
Bolt a 6-foot long piece of 1 1/2-inch angle iron between the front cross member of the bed and the tongue assembly, with 1 foot sticking out each side. I just drilled the angle where the stock assembly bolts go. No problems. Give the angle a coat or two of Rust-O-Leum and it's good for years.
With these few mods, you should now have a rolling trailer frame. Next thing you'll need is a deck for the frame. When you assemble the trailer kit, do not try to make it fold. The trailer is much more solid if you forego the folding option and do not cut the 5/8-inch plywood. Spring for pressure treated exterior plywood so you won't have to worry with paint, rot, or the glue dissolving. As you bolt the bed down, use an eyebolt in the middle of the front edge instead of a bolt to provide a central tiedown point. Later, after the pressure treating chemicals have dried out, you can paint the top of the bed with a porch and deck enamel if you want. Stir in a bit of fine sand for a non-skid surface.
A second piece of angle iron under the back of the trailer frame provides a good outboard position for rear tiedown points, as well as more substantial mounts for the lights. Watch your shins. You have been warned!
Next thing you'll need is wheel chocks. The tubular steel kind can be had in widths suitable for TW tires. They can be fastened firectly to the wooden bed using fender washers to prevent pulling through the bed. If you buy as removable kit like http://www.ebay.com/itm/6-5-MOTORCY...1280&pt=Motorcycles_Parts_Accessories&vxp=mtr
Be sure to provide adequate backing plates under the bed. Tubular chocks are significantly lighter than other options, work well, and not really a good place to scrimp.
Next, you'll need ramps to load and unload your bikes. A simple ramp kit such as http://www.oreillyauto.com/site/c/detail/KEE0/05674.oap?ck=Search_N0771_-1_2270&pt=N0771&ppt=C0373
is another place scrimping can cause serious regrets. The ramp tops have holes to take bolts that then slip into holes drilled in the bed to prevent the ramps from slipping. Cheap insurance. Bike in 1st at idle pulling itself up one ramp, you trotting along beside up the other. Takes a bit of confidence, but easiest way ever to load a TW. Unload by rolling the bike down one ramp while walking on the other, using the front brake to control speed. Easy-peazy. Add a couple 2x4 to the bottom of each ramp, and they fit in the stake pockets for easy storage. Be sure to use clevis pins to keep them from bouncing out.
Don't buy cheap tiedown straps. Scrimping on straps is probably the #1 cause of motorcycles falling off trailers. Many straps sold today have the hooks cut too short. Hooks on right good, hooks on left bad:
Avoid cam buckle straps--they have a bad habit of slipping and dumping your bike. Ratchets are the way to go. Best straps I've ever had--good hooks with safeties, ratchets with big handles, built-in soft ties:
Here you can see the rear angle iron in use:
Note how one of the ramps fits across the back of the trailer.
Note how the angle iron is used to position outboard tiedown points front and rear.
You can see the sideboards and a second ramp being used as a headboard.
The Harbor Freight trailer kit is all you need for two dualsport bikes. Set up as described it will do double duty as a utility/stakeside, too.