I prefer to use two batteries and two switches plugged / wye'd into two different channels. Switches, along with batteries, are known points of failure (unless you use an electronic switch that is normally closed during failure).

You can use two different mAh packs. The total mAh available to the receiver is the sum of the two mAh. I prefer the same mAh packs so when I cycle them I don't have to remember which is which.