If you are Air 2 then you would donate the Air 2, not your primary or only. Well, it could go either way but if the Air 2 diver is using a short hose primary, then he/she will be handing off the Air 2, if they are using a long(er) hose 36 inches plus, then they could certainly donate the primary and themselves use the Air 2 for a direct to surface ascent. Air 2s and buddy bottles, pony bottles and auxiliary bottles gets off into another world and is not generally taught as a primary dive configuration by any abc agency, sport or tech oriented.
I do not have a problem with either way, long or short, primary or secondary donation or handing off a bottle (Air 2) as long as you and your buddy understand each others system and if you have an insta buddy there is a discussion between each their expectations for sharing air. The main reason for the buddy system is diver (and air) redundancy, as long as your system and training and pre-dive discussion with a buddy accommodates that potential need to air share, good with me. I am not a believer in only one way, my way or the highway, one best way or in particular standard configurations being force fitted to every dive and diver.
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