Actually, the Air2 was designed as a backup regulator. In case of emergency, the diver switches to his or her Air2 regulator and donates the primary.
https://ww2.scubapro.com/en-GB/HKG/regulators/products/air2-5th-gen.aspx
For DSDs, we as dive professionals are managing the air consumption. Thus, the student diver should
never have an out-of-air scenario. Additionally, ascending with an alternate air source is not a skill taught in PADI's Discover Scuba, so kitting them with an entire extra hose and regulator is more of an entanglement risk, an additional point of confusion, and a rather useless waste of dive gear.