Everything can be done. But it is definitely an high risk situation, which should be avoided. We should teach and plan for maximum safety, not for risky situations like this...I disagree. Another poster has already stated that he took his buddy's Air2 (and had control of his buoyancy). It's not convenient, but it can be done.
If there is no secondary available, of course we will share the only available primary.Let's flip this around. If you have an ooa emergency, and your buddy offers you his primary, are you going to refuse that as well?
It actually happened to me, the buddy was my wife, and at the time we did only own one reg each.
We were at 30m inside a small cavern. Sharing a single reg with a short hose was not easy. We made it, but after coming back we immediately purchased other two MK5+109. It was july 1979.
After that incident, we did never dive without an easy to donate secondary.