I had two solar chargers on a 5 ton and the batteries would not seem to stay up. So I took out the batteries and charged them individually. I left the solar chargers disconnected since I could not get a voltage reading and the batteries seemed to stay up. I don't know if they are now however, since I sold the truck. I was never convinced that such a small panel could or would produce above 12 volt and cannot verify that they work....at least all the time.
Unless you have a 24v charger, you will have to charge them individually. I tried leaving them connected in their original series/parallel connection (hooking to only one battery at a time) and seemed to have mixed results with at least one, not always the same one, showing a low voltage. When I disconnected the center connection, they seemed to individually charge consistently. On ones under about 10 volts, since I'm not using a charger for glass mat batteries, I hook a conventional battery in parallel until they come up. Once up, the battery charger (without the additional battery) seems to work fairly well. But I measure charge time in days now, not hours.