The one way you could do it is by keeping them electrically separated from each other. Separate chargers, powering separate loads and have NO means to interconnect them, ever. That would really complicate the electrical system in your trailer, so the question is why go to all that trouble? If you really NEED the added capacity, just add one more Li battery to your upgrade.
I just upgraded to LFP batteries and simply sold off the 3-year old lead-acid ones on Craigslist. There's a buyer for almost anything out there. I even sold a used marine toilet one time... so many responses, I could have sold a dozen of them.