A similar question was recently addressed in another thread by Heartland. Basically, they can not endorse any modifications to the Lippert frame. However, it was interesting to note, they did say the Lippert frame they use on their trailers are very strong and in theory should support a hitch.
A number of 5th wheel makers are now beginning to offer 2" class II or III recevier hitches (e.g, Crossroads, Keystone). These are intended for a bike cylce carrier or a very small trailer under 2000 lbs with a hitch weight under 300lbs.
I have double pulled for the last ten years and plan to do so with my new Sundance. The key is not to pull anything that is too heavy, or too long. To be safe, the 5'er rear hitch must be rated for at least twice the weight of what it will pull. I pull a 18' aluminum boat or two, 4 wheelers on a small trailer. Based on my experience, the key is not going too fast ( especially down hill); keep the total weight of the rear trailer to less than 1/3 of the 5'er weight and make sure the rear trailer has brakes.