For the Air2 divers following this thread (I started this way too, back when): map out in your mind how you will handle a TRUE OOA, where your sole source of both air (after donating primary) and venting your bcd will be your Air2.
1) you and buddy will probably be with your right arms in an Indian handshake, horizontal or vertical. That means only one free hand.
2) your buddy can't add air to their bcd if they were negative at the time of failure, so you'll be establishing neutral buoyancy with the same toy that's in your mouth.
3) you will be venting from your bcd on the way up (as will they), but you are venting using the toy in your mouth (butt and shoulder dumps are usually too coarse for maintaining neutral on ascent).
Will you have to remove it from your mouth to get it higher than the bubble in the top of your bcd/wing? If so, you'll be doing that a lot in the last 30 feet. Will you be venting for your buddy too, because they're panicked or task saturated? Do you really want your reg in and out of your mouth during that time?
That's why I've gotten rid of my Air2/SS1/Airsource3. It also really wasn't more streamlined than a second reg under my chin, since the hose had to be longer than my bcd corrugated hose to give adequate head movement when in use. I want my octo on a necklace right under my chin. I donate either 40" or 7' via my primary. There's something reassuring about having a panicked OOA Instabuddy as far away as possible.
(EDIT: just kidding)
(...while checking ascent rate on my Perdix AI)
(not kidding)
My 2¢.