Double trailers are allowed in all 48 states per federal law. This is the same law that allows 102 wide and 53 ft single trailers. All states are forced to go along with this law to get highway funds.
The catch is both trailers must be 28 ft or shorter and they are only allowed on the national highway system.
Here is the roads that includes in Florida.(all the colored roads)
If you are caught off any of these colored roads with legal doubles; one of the trailers is over 28 ft long; or a 102 wide you can be ticketed. They do allow a 3 to 5 mile allowance to get to and from points of loading or terminals.
Out west they allow triple 28 ft trailers and doubles that include one large trailer and one small trailer. This is over and beyond federal law so the states rule on what can be pulled.
Most if not all turnpikes allow double large trailers.