I have two solar panels - first 30 watt 2 amps second 40 watts 2.3 amps, and two 6v deep cycle batteries in series on my RV. My question is: can I simply use two separate charge controllers (max 7 amps each) and run the neg and pos connections from each to the same pos and neg terminals on the batteries? Or do I have splice the wires and use only one charge controller?
I have an RV that I've done quite abit of electrical work, so I know where you are coming from. To start, let me make sure I understand what you are trying to do. Take a look at this block diagram:
I believe the only information I left off is the 7A max for the charge controller(s). To start, let me bring up the topic of this related question: Charging Lead Acid Batteries in Series. It is not the best idea to charge these batteries in series, but it is commonly done without much issue. However, individually charging the two separate batteries should be better for them. This charging technique is commonly incorporated in more advanced Lithium battery systems. You are almost doing that with your setup...
Your charging sources are not identical which could pose problems. Even though the charge controllers may be identical (6V output), the power sources are not. The two solar cells have different characteristics meaning they will output different amounts of power. It is possible that the charge controller could be set to output a constant current just so long as the input stays above a certain level, but you did't specify the type of charge controller. Most of the ones I have seen are cheap without much concern for proper battery charging.
Another bad point is that individual cell charging systems also handle the load output from the batteries. I don't imagine yours will be doing this.
This is a problem because if the batteries are not charged equally (at the same rate, to same level of charge) their performance will be severely deteriorated.
Another thing to think about is the internal RV converter/charger. This is the device that converts shore power (120V AC) to 12V DC to power your internal lights and fans as well as recharging the battery. The stock converters on most RVs are abysmal at recharging batteries. They basically pump a high amount of amps into the batteries nonstop, causing them to rapidly boil over requiring you to constantly add more distilled water. The best chargers use multiple states: high current, normal, and trickle. During charging times, the converter actually powers all of the 12V stuff, and the current I_RV is reversed through the batteries to charge them.
The reason I bring this up is that you will potentially have multiple charging sources if you have everything on at once: 1 12V charger trying to charge the 2 6V batteries in series, and the other 6V chargers trying to charge the two 6V batteries individually. This will most likely damage the batteries, chargers, or both.
I don't quite know what you mean by this:
Or do I have splice the wires and use only one charge controller?
But it sounds like you mean charging the two 6V batteries in parallel with one charger while they are connected in series to create 12V. This doesn't make the slightest bit of sense. It is possible that you could connect the two solar panels in parallel and feed them both into one 12V charge controller to charge the two batteries in series, taking note of my previous comments about doing so.
Here is some information about the work I did to my RV electrical system.