Attaching 50amp cables with marine connections

I guess I should have added that one of my 50amp cables is 30' and the other 15'. I use the 15' one whenever I'm close enough to the post since it weighs much less than the 30' one. However, I have to use the longer one about 25% of the time so I don't want to destroy either one. What I'm trying to figure out is how I can connect these two rather than buy a third cable to use as an extension.