I made a homemade power source for a portable fish finder, light, USB chargers, and cigarette lighter adapter. It's working great, but the offroad light really eats through the batteries. It might last 4 hours or maybe a bit less. So I ordered an identical battery with the thought of running them in a parallel situation to make it last longer. However, now that it's arrived and I've done some googling on batteries, it seems like charging them in parallel is not advised, and also once one starts to not recharge to the full level, it may damage the other battery. It's very easy to switch from one power source to the other inside the box, and the batteries themselves are relatively cheap ($20ish).
My question is this - is there any appreciable difference in battery lasting strength (amp hours) between running them in parallel versus one at a time? IE, will it stay charged longer at a usable level if I run them in parallel versus running one down to nearly empty and then switching over my power cables?