Originally Posted by smanitou
... do I need to keep them together to charge as 12 volt or separate and charge as 2 6 volts?
The choice of series charge at 12 volts or parallel charge at 6 volts is entirely up to you. If you already own a battery charger then use it at the specified voltage. Since you are only providing a little trickle of juice ("trickle charger...") almost any charger will do as long as it is reasonably well regulated. (In other words, the charger will have instructions indicating it is suitable for a long term maintenance charge.)
But I also agree with other posters that if your batteries are in good condition and charged to begin with, then you only need a few hours of direct sun on your solar panel - per week - to keep them charged. However, be absolutely certain that all loads are disconnected during the winter. A good multi-meter is your best friend for this determination.