You are right to a point. But when two batteries are hooked together they do become one larger battery. But they do not double your time you can use them. you will get a longer service life out of two batteries hooked together only if you compare it to how long a single battery would have lasted. But it will be shorter then if you would have just used one of the batteries then the other.
Batteries Equalize themselves when hooked together. So two Batteries that are hooked together will only be as strong as the weakest battery. Because when the weak battery starts to go dead, the strong battery will have to not only do it's work but also the work of the weak battery. Just like when you try to jump start a car whith a dead battery. By therory, hooking your car battery up to it is all you need to do to make the dead battery good. But sometime you will still need to let your cars altinator charge the dead battery for awhile before you can turn the dead cars motor over. that is because the weaker battery is drawing the amps in a attempt to recharge itself. So, instead of getting all 650 Cranking amps to the starter, you only get about half or less.
Now for the charging. Scott is correct in that amps used will determine how long the batteries will need to be charged. but compared to a single battery that is dead. a double battery will take longer to charge back up to full power. What I mean is that if you had a dead group 27 battery. It would take 6-8 hours to recharge compleately with a 10amp charger. Now take two group 27 batteries and hook them together. If they were dead it would take 12-14 hours to get them back up to full power. Because the two batteries would be sharing the 10amps. What it comes down to is whether you have the time to recharge 1 or both batteries before you need them again. That is why I recomend a charge with more than one bank. that way you can charge both batteries at the same time with both batteries getting 10amps.
As far as the batteries being the same type, Let me clear that up too. If you are just trying to charge them at the same time, then it doesn't matter which type they are as long as they are both 12volt like Scott said. But if you plan on using them hooked together they should be of the same type and size. Due to the Equalizing, if you hook a smaller battery up to a larger one, the smaller weaker battery will discharge faster making the larger one have to work harder to copensate. thus shortening its usable work time. Rmember, Deep cycle batteries are different then Cranking batteries. Cranking batteries have high Cranking amps but relitavly low reserve power. so if you try to use a Cranking battery and a deep cycle together. the Cranking battery will discharge faster then the deep cycle, making the deep cycle do more work.
I hope it all makes sence.
John