Rather than repeating this as a fact, can you provide some justification as to why you think that this is so?
It is simple. There are several didactic organizations, such as Fips/Cmas or Bsac, and professional training schools, such as Italian Firefighers or Istituto Rossi, where the rule is to never, ever remove a working air source from your mouth for helping another guy.
So if you decide to give air to someone, you cannot give your primary, you must use your secondary. The Air2 cannot be donated, so it is not considered a valid secondary. A true secondary is mandatory.
I understand that in some didactics or organizations donating the primary is allowed, perhaps even suggested (despite the Bsac study proofs it is less safe than donating the secondary).
But please understand and accept that there are organizations when donating the primary is not allowed.
I already explained this: I am a firefighter. When a firefighter helps someone, he should never give away his air source.
If he does not have another suitable one, nothing is donated.
So if I was diving with just a primary and an Air2, and someone asks me air, I simply must refuse.
When working as a divemaster or instructor I always carry two secondaries: one for me, the other, on left shoulder and with its own first stage, for students or customers.
In such case, I find very useful to use an Air2 as MY secondary. But I must have another secondary on the "wrong" shoulder and with longer yellow hose ready to be donated.
