There's no problem with charging batteries in parallel, especially identical batteries. Obviously it takes about twice as long as charging one battery, since you have twice the battery capacity, assuming the same state of charge.
If you have power where you store the trailer, you can simply plug it in and let the converter keep the batteries charged, which would be easier than toting them to the garage and hooking them up to the maintainer (and eliminate any possibility of hooking something up incorrectly sometime). You could also just hook up the maintainer in the trailer if you didn't care to have the converter and other parts of the trailer electric system live all the time for some reason.
