If you really, really want the redundancy, you can use two separate receiver batteries and two switches. Just plug the second battery into any un-used receiver port or connect it to an occupied port using a Y-harness. Doing this with two NiCads or two NimH is a lot cheaper than all the lipos and regulators. Also, I may get some disagreement on this, but I have seen lipos fail and I have seen regulators fail far more often than I have seen problems with NiCad/NimH batteries or quality switches.

The second battery will add some weight though so I generally only do it in larger airplanes where it isn't as much of an issue. For smaller airplane like a .40 size, I usually just run one good 5-cell battery, generally 1500-2000mAh.<br type="_moz" />