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My hard and fast line of when it's not acceptable is when you still have 2 second stages on your backgas. I would not accept that for an Intro to Tech course nor would I accept it for a solo diving course. If you have a sidemounted or bracketed bottle where it was your secondary second stage, and your only secondary second stage, I would accept it as an alternate configuration for independent doubles. That is not what people think of when they think "pony bottle" and that's why it gets weird when you look at the definition of bailout bottles, safety bottles, and pony bottles. I do not believe that any bottle smaller than 30cf is of sufficient size to be considered a properly redundant configuration though, and if teaching those classes would not accept anything less than 30cf, ideally 40.
I think you would be hard pressed to find any technical instructor who would accept 3 second stages on 2 tanks for any sort of technical class or technical dive or any that would accept a bottle smaller than 30cf.