Outside rails can share the common. The only time outside rails would need to be isolated is if using track activated accessories (crossing gate, crossing flashers, block signalling, gateman, etc.) with a section of insulated outside rails.
EDIT: I didn't mention wiring for DCS, since I remember from previous posts that Vin is not planning for that, but if one were wiring for DCS, then I'm not certain what the correct answer would be.
