First, you can run that 8.4v ign. without concern, directly from a LiFe/A123. The 30cc uses an older module that doesn't like much over 6v.

Second if you want to go batshare, go for it! But 2 batteries is twice what you need, and you already know that so I'm not going to try to talk you out of it.

The opti switch is probably going to plug into a jack on your receiver that you are going to control with your landing gear switch. The Rcexl opti comes with directions that once done, will allow you to pull the power for your ign. module from the same place. Otherwise, you can pull power for the module from a Y harness placed anywhere in your wiring. Just pick a place that's handy.