Can a CFII log Dual Given if flying as SIC?
It’s your logbook, so you can write in it anything you want. If a future prospective employer sees somebody doing what you described, the chances of being hired would fall drastically, since nobody in their right mind believes that the FO as PM is providing meaningful or valid “instruction” in a Part 135 or 121 environment to the Captain. If an FAA examiner sees that sort of time claimed as PIC to meet some requirement, I doubt the reception would be any better.

But, you go ahead &amp; do what you want to do, since you will be the one who lives with the consequences. I think it’s a terrible plan, and I couldn’t see wanting to hire somebody who did that, not only because the PIC and Dual Given number are lower than shown, but also because of the brazen duplicity involved.

It’s playing with fire, and potentially risking a one-way ticket OUT of the airline industry. Which, for somebody who would actually do such a thing, would be the better answer.