What is the best method of connecting two deep cycle batteries together and have power charged/drawn from the?
When connecting two batteries in parallel, the “purist” will want you to connect to the Pos of one bat and the Neg of the other bat.
Connecting both of your charging or load connections to just one of the batteries would be perfectly acceptable – provided – the cables that connect it to the second battery had no resistance. In the real world, there will always be resistance in the wire and terminations. By making the Pos connection to one battery and the Neg to the other, current into or out of either battery will have to flow thru one of the interconnecting cables and both will see the exact same load.
If the interconnecting cables were only 1 foot long and made of very heavy cable (like a #4 battery cable) and your load was small (just a few amps), you could connect both of the load wires to the same battery and never know the difference. If you were cranking an engine (very heavy load), this would cause some drop in the interconnecting cables and the second battery would not provide its share of the power.
Since the batteries will be next to each other, there is no reason not to connect the load or charge with Pos to one bat and Neg to the other. If this were a camper with the second battery located remotely, this may not be practical and you just live with it.
Note: When connecting batteries in parallel, make sure they are identical. If there is a difference in the battery (even different concentration of acid in the cells), the batteries wont play well together. If you have not yet purchased the batteries, might I suggest that instead of two 12v batteries that you get two 6v batteries and wire them in series? This eliminates the balancing issue completely. 6v Golf-Cart batteries are designed for deep-cycle use and would likely be a good option for your solar power system.