Here's my take on your situation. I would scrap the old battery, and just leave the dual set up in service. It would be my luck that I would blow out my back moving the setup back and forth. Also, there's storage of two batteries, and remembering to charge them as well. I doubt you'll notice the extra weight of the dual setup.
Also, for me at least, we often decide at the last minute to head out. Switching batteries would be one thing less to remember to do.
2000 Cherokee, LEDs, solar, inverter, etc. '07 Tundra 5.7L DC-LB, modded as well.
2000 Cherokee 29BH with 6V batteries, LED lights & 400 watts of solar power, flipped axles and raised. 2007 Tundra 5.7L DC-LB with lots of mods. C-co, 8/158th AVN Maint.