While the C10 can safely haul.about 250#, to a questionable max of 300#, as long as the weght on the tongue at the ball is balanced to give 20# at the ball, its more of an issue of C/G on the actual loading more than just a weight thing...
We have experimentd with home made trailers, and heavy loads, but the problem is the C/G really can't be more than 8" above the top of the tire, based on a 10" rim tire...
Straight line is fine, but once you apply the torque, of weights above that, on the frame and wheelbase, it rapidly get squirrely, and downright dangerous as this C/G rises.
So a 300# bike, + the actual trailer weight(approx 150-175#) will easily flip, and or cause the bike to loose control due to weight transfers in turns. And you still have almost 500# of force pushing when you try to slow down... not a good ratio of pulling bike mass vs pulled mass...
I made some 3D solid Cad models during design stages, and even making a custom trailer, with a "drop axle" to allow the tire mounting rail to locate so the tire sits about 8-10" max off the ground, side forces were still too great to chance it.
The C10, and C14, both handle fine, with a 170# load, in the Harbor Freight "toter" trailer, but if you excede the height rule, even on that trailer, things get out of control fast... case in point, I placed 180# of sandbags in the trailer, centered over the axle... and hauled it thru twisties.. never had a glitch, even braking in turns, downhill...
Then, placed 100# of sandbags in trailer,.same location, and 70# in a milk crate, tightly cinch strapped to.the top of the shell... it was so dangerous I cut the testing short within the first 3 miles,.never even tested braking on downhills, and simple curves in the road exhibited so much stearing and trailer movements, and out of balance feel, I couldn't risk it further...
Mind you, this was on my C14, with far surperior brakes, stiffer frame, bigger tires, and more modern design.. and using a really robust trailer hitch which had robust bracing, and wide and even, strong hard point mountings compared to wht is on a C10.
Feel.free.to.create.something, but I'm just giving a heads up, before anyone dumps a bunch of $$$$ to find what I found... a bike sitting high,.with the handlebars tied down to the frame of the trailer, even a low C/G trailer, is one heck of a lot of leverage, even tho the actual "up top.weight" isn't extreme,.the leverage is the killer, making the trailer want to lift a tire... and flop over.
So while Daves sidecar rig, by design, and its mountings to the "hauling bike" work great, and the C/G is well withing the safe "sweetspot", and also off to the side, the bike.hauling bike thing.... mmmmmmmm not a reccomneded adventure.
Be safe.
I also.did some.designs for a type of small,.2 wheel thing the towed bikes front tire could mount on,.kinda like a car carrier "front end dolly", and even that didn't.give me any confidence, seeing as it takes some robust starpping and shock absorbance, along with dampers to prevent sway... and after all that,.you are still.hauling the towed bike,.with its rear tire burning on down the road, putting wear on it,.the chain/shaft, and bearings associated with it. In the end I gave up on that one also.