RE: single battery possible?
For small airplane applications (60-120) where this discussion makes sense, one could use lipo + regulator for the receiver and/or ignition, to save a few oz.
Thunder Power Pro Lite v2 790mah: 1.6oz
Thunder Power Pro Lite v2 480mah: 1.2oz
Pick the regulator that has the proper capacity rating (e.g., 5A for non-3D fly).