MoS: Simple response to your question is, Yes, it is OK to share air and dive.
Not so simple response: IF your profile is correct, you don't have a lot of diving experience -- less than 100 dives over a three year period -- less than 35/dives/year. Because you write that you are a "hoover" -- and from the difference between your buddy and you it would appear you ARE a hoover -- it is likely your general diving skills are pretty typical of the "casual open water diver." This is NOT intended as a slight but just as a comment.
So what are the diving skills of such a diver -- moderate (at best) buoyancy control, relatively poor horizontal trim (i.e., diving at a 45 (or more) degree angle) and some use of the hands for "stability." IF this does not describe your diving style, please forgive me for the assumptions -- but if it does (at least in part), then before you use the "underwater tank equalization procedures" you really need to get your skills buffed up -- and there is a real reason for this.
When you are diving attached to another person, which is the "underwater tank equalization procedure," both of you need to be able to really control yourselves in the water column. And not only control yourselves with relation to the bottom, but be able to control yourselves with relation to each other. That is very hard to do without good (not moderate) buoyancy control, good trim and/or good stability. If you are connected and swimming along with your buddy and she stops to look at a pretty fishie -- will you be able to stop too or will you swim off and pull the reg out of your mouth? Or what if YOU stop to look -- can she stop too without swimming the reg out? Are the two of you able to maintain your position in the water column within a 2 foot range at all times -- because if not, you will likely put a whole lot of stress on your mouth if she goes down and you go up. Of course an added benefit of having better control of yourself in the water column (better trim, no hand movement) is that you will use much less air yourself!
Why do I know this? Because my buddy (my wife) and I use the "underwater tank equalization procedure" routinely -- at least routinely when diving in the tropics on AL 80's. Even with a 7foot hose on the donated regulator, that doesn't give the two of us (or you) a lot of wiggle room regarding where we are in relation to each other. But, fortunately, our general diving skills are such that we can swim like that for as long as we want -- stopping when we want, going up, going down as we want -- even moving around and taking pictures -- all without thinking about keeping together and without putting a strain on my mouth.
For what it's worth, I would only do this with someone who has a 7foot hose because anything shorter just doesn't give enough freedom of movement for each diver.
Also for what it's worth, when we do this, I'll generally go on her air when we are at the deepest part of the dive (I might have 1800 psi, she'd have 2400) and I'd stay on her tank for five minutes or so -- or until she was down to about 1600 psi. We both ALWAYS have sufficient gas to get us both back to the surface using a standard ascent profile from wherever we might be.
While I respect (well, sort of) people who write that sharing air should only be done in an emergency, I think those people are dead wrong -- and quite frankly I decided NOT to do a DM with an instructor in no small part because that was his thinking. What that showed was a lack of understanding of the overall situation AND a lack of comprehension that it is actually a GOOD thing to periodically share air with your buddy while diving. Why? Because it trains you that sharing air is NOT something special -- it is merely doing "tethered, coordinated flying" through the water -- nothing more, nothing less. Think about it -- IF you routinely share air, swim around, look at the pretty fishies -- how much anxiety would then be generated when your buddy comes over to you and wants to share air "for real?" Probably very little -- you are used to swimming "tied together" so the reason for it is pretty much irrelevant.
I think the reason SOME people think it is such a big deal and should only be done in an emergency is because of two main reasons: a. The gear that is used -- swimming along normally on a 36inch hose is really tough; and b. Most divers, quite frankly, can't control themselves in the water column well enough to "dive in formation" for any length of time. The solution to this is NOT to say -- Don't do it except in an emergency -- but, to the contrary, to have gear that allows one to donate comfortably AND to practice skills such that diving "in formation" becomes trivial.
OP -- work on YOUR skills and then have fun diving in formation and looking at the pretty fishies -- it is a lot of fun.
