They make float chargers specifically for your needs. If the batteries start in good condition one should be able to keep both topped off. For about the same price you can buy a smart charger that will bring up a battery that is partially run down and then go into the float mode when the battery is fully charged. I use one of the smart chargers on an old tractor that has a bad generator. It stays on any time I'm not using it and floats most of the time but brings the battery back up after I have been using it.
I also have several vehicles that I don't use in the Winter. Instead of using float chargers on them I just top off the charge with my smart charger mid Winter and in the Spring. Sometimes just the Spring when I forget. Works for me.