or are you talking about two batteries connected in parallel which provide both cab & hab services.I've never seen this in a professional build
I've seen this in boats many times and my present boat has such a setup. In fact they make dual purpose batteries for this. But it wouldn't really be suitable for a motorhome.
But the way I read it is that the OP wants to add a second leisure battery in parallel l to the existing one. But I could be wrong.