If you're an active diver, you most likely have an alternate air source that you consider sufficient for your needs. Since our last evaluation of alternate air sources in January 1995, there have been a few improvements, but no significant breakthroughs such as those we've seen in primary regulators. At the same time, alternate air sources still receive mainly lip service. Sure, they're required during training, and by most dive operators. Sharing air is often not practiced, and the alternate air source is likely to be a regulator of lower performance that has not been well cared for.
Chances are, neither technique nor equipment is up to the task.
It should be securely attached so it does not swing free and get damaged by or cause damage to the environment, yet it should detach easily when needed. It should be easy to use. It should be completely separate or have a hose that is long enough so that divers can share air without being so close that their movements are inhibited. If the donor is passing a second stage to the needer, that reg should have a longer hose, regardless of which one it is, the primary or the secondary. It is often best to use an alternate air source and primary reg from the same manufacturer.