ccdave,
The Y-harness would in fact allow you to have two batteries. But most people will also want two switches in case the switch fails. The second battery/switch is identical to the first and is plugged into any extra receiver slot. This is most often seen on larger high $$$ planes where the extra weight and money isn't an issue.

Personally, I feel there are about 4872 other reasons that are more likely to make your plane crash. I use one battery.

Tom