Double towing - what things to consider ?

[Hanr3]

I triple tow a 5th wheel and 16' Deep V boat.
28 states allow triple towing. The entire East Coast, Washington, and Oregon prohibit triple towing. California requires some sort of special permit. All states have a length limit restriction. Some states require the towed vehicle to be a 5th wheel, but not all. Some allow teh first trailer to be a bumper pull, mostly out West.
I live in Illinois and our length limit is 60'. That isn't entirely true. Illinois allows 80' on the Interstates and State Highways with a few ramp exceptions around Chicago, East St. Louis areas. There is a 5 mile grace for fuel and to the campers storage facility. Most of the restrictions are in cities/towns and their roadways.

To answer some of your other questions.
The main things to consider is weight.
Can the truck carry the weight of the 5th wheel pin which can be up to 25% of the 5th wheel total loaded weight?
This includes fuel, passengers, cargo, etc in and on the truck. Can the truck tow the combined weight of truck and both trailers.
Can the 5th wheel tow the boat weight? 3,000 pounds is common.

I'll use my situation as an example.
I triple tow with a F150 3.5L Ecoboost Extended Cab with standard bed (6.5') 2 wheel drive.
KZ 231RK 5th wheel, maximum is 7,000 pounds fully loaded.
Sylvan 16' Deep-V fully loaded on trailer weighs 2,300 pounds.
My truck in this configuration has a GVWR of 6,900 pounds. truck fully loaded weighs (included 5th wheel hitch, gas and passengers) 5,400 pounds. Leaves 1,500 pounds for the trailer pin weight. Actual pin weight is 1,400. 100 pounds to spare.
My towing capacity is 10,300 pounds- Camper and boat weigh 9,300 pounds. 1,000 pounds to spare.
My trucks GCVWR is 16,900 pounds. Fully loaded all three weigh 16,200 pounds. 700 pounds to spare.
Total length of my rig is 59.25'. I have 9" to spare to travel anywhere in the state.

Let's talk fuel economy.
Truck empty averages 24.2mpg on the interstate.
truck/boat 17-18mpg.
truck/camper 10-13mpg.
truck/camper/boat 8-10mpg.
I never have trailer sway from passing vehicles and I get passed by almost everyone. When towing I drive 65 maximum in 70mph zones and the speed limit in all other zones. There is no way I can outrun anyone, nor do I try. Less stress for me to let them go around. I have a very comfortable ride. Never feel like I am on the edge, or white knuckled. Beware, not all 1/2 tons can tow a 5th wheel. I mentioned the specifics of my truck for a reason. That combination gives me the ability to tow a 5th wheel. Be carefull assuming a 3/4 ton will be better. I can out pull almost all small V8's in 3/4 ton trucks, especially if they are crewcabs.