Bottom line it comes down to common sense. if its legal in your state then use common sense, don't put a larger heavier trailer behind another one. Put the heavier one in front/behind your tow vehicle, Don't exceed overall length laws, have brakes if required, have the appropriate licensing, Make sure you have enough truck and drive with common sense! Don't commit to pulling in to an area you might not be able to turn around in.
Use a MAP! Plan you trip ahead of time! (Might need to actually go for a ride)!
Also,, the use of a WD hitch is for the tow vehicle and you should never use one behind your trailer because it doesn't work that way!
Double towing can be done if legal, Many people do it in my state and I also do it! I have a TT not 5th wheel and it works just fine. I fish with a friend that has a 5er and he tows a really nice Pontoon/Patio boat behind his 5th wheel. We use the patio boat as the "Mother ship" and can tie up to it and relax and party with them!!