I love the hubby line too, and i think by the sounds of things, if you even tried to explain to him what you were doing, he probably wouldn't understand anyway.
If he asks more questions when he comes back to do the work you could always suggest to him that if he has a problem installing the extra line that you'll take ALL your business elsewhere a la m0rtal's post.
or you could say that it's fine if he doesn't understand how to put the second line in - the other company's workmen shouldn't have a problem.