Such a control circuit is possible, but a better idea is to leave the panels fixed in one configuration and deal with the resulting change in voltage with the right kind of switching power supply. If you want to get fancy, you could even implement maximum power point tracking, or something reasonably close.
The switching power supply can't make more power than what the panels produce, but it can convert (with a little loss) from whatever voltage and current the panels want to produce to a different controlled voltage or current (with the other limited by the available power).
If the series configuration always produces more voltage than you need even in low light, then that's how you should configure the panels. The switching power supply then always makes a lower voltage, which keeps it simple. That is called a "buck" regulator, with much information about that out there.
It would be helpful to say what voltage you ultimately want, and what current range is useful at that voltage.
Added:
You now say this is to power a 12V system, which presumably runs from lead-acid batteries although you didn't say that.
One useful feature of lead acid batteries is that they can take reasonable charge current even when full and still regulate the voltage well enough for most purposes. Given that, a really simple solution is to wire the solar panels in series to get the higher voltage you talked about, which is always a bit more than the 12V battery level even on cloudy days (when light is really low the voltage will be lower, but then there is so little power to be irrelevant), and just connect this to the 12V rail with a Schottky diode. That will not use the panels most efficiently in high illumination, but probably not so bad on cloudy days from the numbers you give.
A buck converter that runs the panels at the best efficiency for the given insolation and then dumps whatever current it can onto the 12V rail should be a bit more efficient. With a decent converter design, the extra loss in the switcher should be more than offset by running the panels at their optimum efficiency.
However, if you have more than enough power in full sunlight and the real problem is when it's cloudy, maybe the dumb series connection (with a Schottky diode to prevent reverse current when dark) will do it. I'd probably be tempted to try that first and see what you get and how efficient the whole system is on cloudy days when it really matters.
You could connect your battery string directly to the solar panel: the voltage is about right. However, your VRLA batteries are sealed, so you should never over-charge them.
Your "24V" panels probably have a peak power point around 36V, and an open circuit voltage around 48V. This is a suitable voltage range for running a "24V" power system, which actually runs at around 28V, charges at 30V, needs around 32V before losses, and needs some overhead so that it works in low-light conditions.
Running a "36V" system at about 42V, with charging around 44V, you wouldn't get very good solar efficiency out of your "24V" panel, and you wouldn't get peak charge current out of them, so they would not be 200W each (400W total) except perhaps under exceptional conditions when cold and new, running into a flat battery.
A normal charge controller can "handle" this situation, that is, it won't die, it will charge your battery, it won't overcharge your battery, it won't destroy the UPS. *Assuming the charge controller voltage levels can be configured for sealed batteries
The question is, can you "handle" the fact that you won't get 200W (each) out of your 200W (each) panels if you configure it that way?
Having said that, it's a UPS. You may never use it. You probably won't need 400W. And it's a stop-gap: your expectations don't have to be high.
Best Answer
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:
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.