From a technical standpoint, what are the drawbacks/concerns with normal 250/350 class trucks doing a double tow? Sway control? Traction? Wheelbase? Towed weight vs Vehicle weight?
I've never done it, so I have ZERO first hand knowledge of the situation, but from a technical standpoint there isn't anything that jumps out as a "Holy crap that won't work" issue.
Genuinely curious, not trying to be combative/obtuse about it.
I didn't take it as combative, as it's only my opinion, and I'm willing to accept that I may be wrong too.
To address your comments, I don't consider a 350/3500 truck to be a common passenger vehicle. The 250/2500 I view similarly, although I see them as common when someone is looking for a purpose, such as towing a 5th wheel or a larger boat. My neighbor had a 250 (and recently moved up to a 350 when his 250 finally crapped out on him). He does/did dock installs and LOTS of towing of equipment, cars and materials. Seems to be the common theme with anyone moving up to a 250, they see a definite need above and beyond what I would consider the typical truck - an F150/1500.
There's plenty of people who don't know the difference (or care, or consider it) between that step up. I used to rent equipment in a past career, and often, people would show up with a 1500/150 when they were explicitly told on the phone prior to their coming in to the store, that "An F150 will not haul this, and you have to have at least a 1-ton to be able to rent this, we will not rent it to you otherwise". Then I'd get bitched at because "it's got a hitch that'll handle that, I don't know why you guys are being such jerks about it". I also did landscaping, and this would happen too - I can see your springs are bottoming out, and you've got a truck bed FULL of stone. That's a 3/4 ton pickup, and you've easily got a ton or more of stone in there. Why anyone would load that is beyond me.
These things are why my personal opinion is that a 250/2500 or larger isn't a "common" passenger vehicle, but I realize that others can/will view that differently, and that's fine with me - it's simply not my view, and we can coexist happily in the same world.
The main concerns I have are some that you mentioned - braking, sway control, load weight AND distribution, sidewind or wind loads in general, driver experience, traction (which seems like it'd be tied to wheelbase and load/distribution/braking). Double towing isn't common for a reason. Many folks have problems just towing a single item behind them. I've personally witnessed a single equipment trailer get loose on a friend and eventually take him off the road and backward. He was running a 2500, and easily puts on 200+ miles per day towing that trailer. The typical camper/boater doesn't have as much experience, although I realize and acknowledge people who double-tow very well might have lots of experience. To me, it's a risk/reward that isn't worth it. People have lost their lives towing single items that got out of their control.
My tractor trailer has 4 different ways to brake, depending on the severity and need (engine brake, brake pedal, air brake, super-duper pull the lever emergency brake), so it doesn't seem unreasonable to think some additional method of braking assist would be wanted by the person looking to double-tow, but just like people pushing the limits of what they deem to be reasonable use for a 1500/150, I know there will be people who blow it off because of reasons. These can help with sway control too, but they're not infallible - everyone on here can see pics of a truck/boat combo that's lost on the side of the road (or smack in the middle of the road).
Your statement kinda is the point of why I personally won't do this - there's nothing jumping out at me that says it won't work, but that doesn't mean I WILL do this. The risk/reward for me is simply too far weighted in the risk column. If I needed both to have fun, there's ways to address it. Rent a boat or camper/cabin where you want to stay at, bring a tent, have the wife tow whichever item she feels comfortable towing while you tow the other, etc etc. At the very least, I can't control the other doorknobs on the road and how they'll drive around me, so that'd be the final item I'd take into consideration before attempting this.
I'm not saying anyone's wrong for doing this, but I don't agree with it, and I'll be the guy giving you a SUPER wide berth and trying to help you manage yourself in traffic if I can do so, since I'm familiar with how people drive/react around larger vehicles or towing a load. The jerks that WON'T do this far outnumber me on the road, to the point that I'm comfortable with my opinion even if there's disagreement with it.