PC12Driver said:
Nope. Lots of miles pulling a bunch of different trailers in all conditions, but no doubles.
Is this the part where you try to tell me that towing two bumper pull trailers is perfectly safe because you've pulled them and haven't had any problems, and I don't know what I'm talking about because I haven't?
To a degree, yes, this is where I am going to say something to that affect, while not trying to be a total smart*****.

In any scenario someone with hands-on practical experience is in a better position to speculate on a topic than someone with no experience. I am not saying towing doubles in any fashion (ball or 5th) is “as safe” as single towing. However, to speculate that just because it is ball to ball it is completely unsafe, without practical experience is making an uninformed statement.
I have pulled single and double for ten’s of thousands of miles and can tell you through “hands-on” experience that ball to ball can be very stable when set up properly, just like any tow setup. Part of that setup means weight distribution and sway control on the first trailer. Part of that set up is that my 2,000lb boat or 2,500lb ATV trailer doesn’t have a lot of directional force on the 7,500lb truck or 10,000lb RV in front of it. There is not a lot of “tail to wag the dog”, so to speak. I have driven in just about any condition, with the worst being the winds of Wyoming along I-25, and my setup has been very stable and I get very little additional sway from the double compared to just pulling the RV in the same type conditions.
As stated in my previous post the only scenario where I personally experience any difference is long downhill stretches on interstate (high speed) highways, such as I-70 dropping into Denver from the mountains. Where I can comfortably go 65-70 with the RV in tow, those speeds get a little more squirmy when double with the downhill forces. Taking the speed down to 55-60 gets everything very stable again.
Again, I am not saying it necessarily as safe and single towing, for that matter towing anything at all is not as safe as driving unloaded. Does that mean even single towing is unsafe? As with anything, without the proper setup it may not be safe, however, if you take care of the details and match all of the components properly (including size and type of trailers) it can be plenty stable. It definitely helps having a much heavier front trailer (rv) than 2nd trailer (boat, atv’s). Through experience, I personally would agree that it’s unsafe to pull double trailers of near weight with any hitch setup (5th or ball to ball, not including big rigs here).