The advantage is redundancy, if one battery or switch goes out for some reason you still have one working. Only way to know you have a problem is with a volt meter if you check voltage after every flight. I have one big Sub-C battery pack that has two wires for separate switches, if a switch goes out it still works, if the battery poops out who cares how many good switches I have.
Everyone figures it differently when to go to two batteries. After 60 size I often do it, not always but often.
There is another thread talking about 2.4 VS 72, think it's titled {why haven't the 72s gone down in price} it's worth a read. I'm just now starting to change over slowly as money starts filling my pockets, very slowly. There isn't anything wrong at all with 72 other then watching out for other 72 people. One thing you may want to invest in is a good AC/DC charger and a good loaded volt meter. A good charger/cycler is something everyone should invest in and a meter is used several times a day when at the field. When I'm at the field I may be flying one plane a lot and as soon as the battery is down I plug it into my field charger and fly something else until it's charged back up. During a BS break I plug in my TX, we all check our planes but I tend to forget my TX.