It all depends on the capacity of the trailer, which is a combination of frame length & capacity, axle capacity, no. of bunks/rollers, tire size and load range, etc. If the trailer is rated to handle more than you could ever load on it, then you're fine.
My 20' Grady sits on a 1982 single axle, 32 roller EZ-Loader. It's rides on 16.5" rims and has massive 12x2" drum brakes. The trailer easily handles the weight and tows like a dream. Saves me $$$ on tolls too as it's only a single. [img]/images/graemlins/smile.gif[/img]
It looks to me like your trailer is actually too small for the boat. Adding a second axle won't do much for you in this case. I like a trailer where the frame extends almost to the end of the boat. That helps take some stress off of the front of the trailer frame. My suggestion would be to sell that trailer and get a properly sized one...single or double axle. You may have trouble finding a big enough single though. Manufacturers just don't seem to be making heavy duty singles anymore.
I don't think I would put a rig more than 17' on the trailer you have now.
Good luck. [img]/images/graemlins/smile.gif[/img]