but might 36v from a pair of panels damage the actuator circuitry?
So here's the deal. Lead-acid batteries look electrically like a voltage source/sink with a small series resistance, with the voltage level a function of state of charge. 2V/cell (there are 6 cells in series in a 12V battery) is nominal, and if I remember right, their open circuit voltage is something like 1.9V empty, 2.1V full. That covers 90% of their behavior.
Considering that, the "1W@18V" spec of the solar panel isn't going to be able to "win" against the battery, and the solar panel's voltage will be pulled down to battery voltage, delivering probably 0.055A (=1W/18V) at whatever the battery voltage is.
When a battery gets completely full, however, its series resistance goes up dramatically, and the voltage goes up, until there's enough voltage to start electrolysis of the fluid and you get H2 and O2 generation at the terminals and loss of the electrolyte. A lead-acid battery, depending on the type + manufacturer, has a certain recombination rate of H2 + O2 => electrolyte that it can handle; if you electrolyze at a higher current than that, it leads to permanent electrolyte loss (+hence capacity loss)
So there is a safe current that can be delivered to a lead-acid battery continuously, where its own self discharge due to electrolysis balances the charging current. It depends on the manufacture + construction. I wouldn't feel worried about a C/10 or C/20 rate of charge (where C = the current needed to discharge a battery in 1 hour). Garage door batteries are probably > 1Ah capacity so you should be safe with 55mA charging current.
HOWEVER -- I would probably put a (zener diode and resistor in series) in parallel with each battery, the zener diode being about 14V and resistor being maybe 10 ohms or so, so that it keeps the battery terminals from getting charged too far.
Also: if you can, wire each solar panel to each battery (and keep the diodes), rather than the pair of panels in series wired to the batteries in series -- i.e. try to connect the center taps. By doing so, you'll charge each battery independently. Otherwise, what can ruin battery life is if the battery voltages diverge -- the one with the higher voltage will tend to get overcharged, while the other one will tend to get overdischarged and not completely charged.
Taking the question at face value, ALL the answers given are wrong [ :-) ] and your question and a subsequent comment do not match.
You say you have 4 x 12V batteries to make a 48V supply.
Your diagram implies (but does not state) that the panel is 24V rated.
In a comment you say
I don't think I know the PV array's knew voltage. I know it produces up to 18V in full sunlight though. – Wurlitzer Jul 10 '12 at 23:00
An 18V O/C panel is 12V rated, not 24V as per diagram.
Cheap and nasty solution:
Use a relay (or relays) to swap the panel between batteries. Either two positions if 24V panel or 4 positions if 12V panel.
A simple sequencer will be required to control the relays.
Very fast swapping rate reduces relay lifetime, and controller will take a time to settle so "dead time" % is higher.
Very slow swapping rate increases risk of imbalance.
If you swapped every minute or even every 5 minutes the long term differences will be minimal.
A good "real" solution is to use a 48V panel - or 2 x 24V or 4 x 12V panels in series.
Also suitable would be to use a 12V to 48V converter. Some MPPT controllers are available which will both up-convert the input voltage and also optimise loading to maximise panel power output.
Best Answer
Should work. Cheap charge controllers are a bang bang MOSFET switch, so either the battery is straight on the solar cell, or not. MPPT controllers have a switch mode buck converter; two of them on one panel will get very confused.
Once the battery is full, the charge controller switch will be mostly off, so the other charge controller can use the solar cell.
The danger might be that if both batteries are low, both controllers will come on together, shorting two batteries together and damaging the controllers. You could check for diodes or add them. They must go before the controllers, not between the controller and the battery, as it needs to sense the battery voltage.
The open questions are 1) are there already diodes in the charge controller, and 2) what does the controller do when the solar voltage collapses to ~14V (when the other one shorts the panel to the battery). It might panic and not charge.
It's worth asking the supplier about both of these, I'm sure they've had these questions before