How many divers that are not DMs or instructors actually have donated in real life, I wonder? I was involved in a buddy breathing scenario in the late 1980's before the people I dived with as well as myself started using octo regs. Prior to that the last time I did it was in the pool in 1968. The only time my octo reg is used is testing at the beginning of a dive. It is far and way the most expensive and unused piece of gear I own.
As I posted earlier I remove the octo from my primary when using independent doubles because both sec stages have hoses long enough for OW donation which is all I need. I still call one primary and one secondary. The difference for my rig is the primary is on the left and supports my wing inflation. The tank/reg on the left is secondary and has only a SPG and the sec stage.
I share gas many times a year just as practice, or to share all available gas (1 diver on doubles, 1 w/ single tank, etc) since that also serves as air sharing practice. Yeah germs, but since I’m married I imagine my exposure is nothing compared to normal dating life, let alone a single city dweller’s (I live in the boondocks) commute sharing buses/subway handrails with hundreds of other people. Sharing a reg underwater with 2-10 people a year is unlikely to be the critical act.