That's the easiest way I do it, and it's one of the ways I teach it. Especially underwater (all of the hoses and shoulder straps naturally fall into the right place while the BCD is over your head. I'm a little surprised you were buddy breathing while doing this skill. Sounds like combining skills to me. Maybe SSI allows for that. I don't, at least not till DM training.
That is not a required SSI skill. You have to be able to do the BCD removal and to be able to share air but not combined. Sounds like the instructor was going beyond the standards.