I often get called because an old gate system has failed and they have the solar panel and battery, which tells me it is probably a 12v system. Most control boards have a relay or two and unfortunately a relay is either 12v or 24v, so what I am saying here is that control boards are either 12v or 24v.
Most 12v swing motors will be fine on 24v so that is not a big issue, but it is not good to feed 24v into a 12v controller for reasons I have just stated.
So why is one system 12v and another 24v. Well it comes down to the voltage and the ah of the batteries.. So the question is... is a single 12v 18ah battery better or worse than 2 x 12v 9ah batteries in series. Well the answer is they are the same. A 12v system draws double the current of a 24v system so it all ends the same.
If you have physical room for a 12v 20ah battery then that is a better system compared to a 24v system. but most control boxes are too small for this so they use 2 x 12v 9ah batteries in series..