Thanks oldspark, In effort not to derail this thread, I've started a new one here: http://www.mp3car.com/power-supplies...ml#post1473267
Yeah, should be fine.
As long as batteries are charging (eg, typically above ~12.8V), they can't dump into each other.
The whole "not paralleling" thing is to prevent a faulty battery dragging down the other and hence damaging that too.
When paralleled for big loads or longer reserve times, that is ok (or rather, it's assumed to be necessary) provided both batteries are in similar condition - ie, no faulty battery or collapsed cells.
The latter also applies to charging - a bad battery can drag the charger down and the good battery can discharge into the bad battery as well. Safety wise, that can be an issue - more current and hence heat to flame the bad battery. And a bigger charger might keep the voltage up, but the bad battery still absorbs the higher current.
Also keep in mind excess voltage - that produces gassing which can be good for maintenance, but not for too long. Hence solar panels be regulated. Many say they don't need it for small panels, but I and many others disagree. Any overvoltage causes gassing. The only time a panel does not need a regulator is when its output is less than the battery's float current (ie - fully charged absorbed current; usually at a float voltage of ~13.2-13.4V) and they can vary from mA (maybe 100mA) to 1A or 2A for most car batteries.
Pretty simple isn't it? (Not?)
But here comes a bit more complexity...
Since solar panels (should!) have blocking diodes (typically Schottky diodes for their lower voltage drop of ~0.3V instead of a silicon diode's ~0.6V drop), it should be simple enough to use parallel diodes - one to each battery from the panel.
Hence batteries can remain isolated (for when not charging - ie, night time or reduced sun) yet still be charged by one solar panel panel.
One problem may be the regulator which is usually between the panel's diode and the battery, however assuming the regulator has a remote battery voltage sensing wire (as opposed to sensing its output voltage), the diode(s) could be moved to the regulator output - ie, between the reg's output and each battery.
If the reg does not have a separate/remote sensing wire, then the battery(s) will get ~0.3V less than they should (ie, a Schottky diode's forward voltage drop). That may not be too bad, though they should be properly charged at intervals if their charge voltage isn't above 13.8V or preferably more (13.8V is not enough to reverse the sulfation built up in a non-full battery).
If the reg does has remote sensing, then a "sensed" battery must be chosen.
I'd sense the battery that takes the lowest charge current (or has smaller loads) as that will have the smallest voltage drop across its diode. Higher currents increase diode voltage drops and it's better to undercharge one battery than overcharge the other (ie, gassing etc).
However, if the regulator is set for say 14.2V, then the bigger or loaded battery could be sensed. That's based on a long-term maximum charge voltage of 14.4V for lead-acid batteries. Hence if the high current battery gets 14.2V through its diode, the low current battery might get 14.3V or which is still ok. EG - the sensed battery gets 14.2V with a 0.4V diode drop, hence the regulator is outputting 14.2 + 0.4 = 14.6V. The other battery might have a 0.3V diode drop due to less current thru its diode, hence it gets 14.6 - 0.3V - 14.3V, still 0.1V under the ROT 14.4V maximum.
Some regulators have adjustable outputs which can be increased to compensate for diode voltage drops (especially if lacking remote sensing), or reduce their output voltage (from 14.4V) to ensure that with the lowest diode voltage drop, the battery's max charging voltage is not exceeded.
If you think all that is tricky, you can imagine why diode isolators fell out of favor for automotive dual-battery setups. Their diode drops varied from ~0.6V to well above 1V at high currents; 1.4V was not unusual.
What do you do when your 2 battery charging voltages differ by 0.5V etc? Which battery do you sense?
[ I tackled many diode-isolator issues in two mp3car threads involving Simbalage (simbalage21?) who was adamant that they were "the only safe" and best form of isolator (and that relays were NOT isolators LOL!). Since then I have confirmed that no emergency services here use them. (Police vehicle's battery isolating relays are controlled by their (Delco?) EMS and regularly connect and isolate. Other emergency vehicles also use relays.) In fact quite a few baulked or laughed at the suggestion. Poor simbalage, taken in by the bullsh claims and half truths favoring wasteful & backward technologies and profits. ]
I've detailed the issues that can lead to a worst case situation. You are likely to find that they have little consequence, especially for small solar installations that do not provide huge currents and hence only have small diode voltage-drop variations (I think even less with modern Schottkys).
For piece of mind, use your voltmeter. (A $10 DMM will do.)
You should regularly check the OC (Open Circuit) voltage of the batteries. (Allow for surface-charge dissipation after charging.)
Don't connect unequal (voltage) or "low voltage" batteries unless you know they are merely discharged.
Ensure that the batteries get appropriate voltages and that the regulator else panels cannot over-charge.
Now and again perform battery maintenance - ie, an equalisation charge etc. Else connect to the car's alternator for a good blast.
And beware warm batteries, that's a sign of overcharging. However, siting in the sun or a hot car or engine bay or housing may have an effect (ha ha).
Speaking of the sun, it has just risen.
I'm off to bed.