Do the people who use the BC incorporated secondaries not typically use a long hose primary?
Buddy breathing off a shared primary sounds like a recipe for disaster, as the OOA diver is almost certain to be panicky. On my recent dive trip the dive plan was that the DM would take the first LOA diver up for a safety stop and their buddy could repair with another diver- as long as the group stayed together. My husand out-lasted me by several hundred PSI and buddied up with another diver who had been just tagging as part of the group without a dedicated buddy. I was just about done with the safety stop when my husband joined me, so I hung out for another 3 minutes and we surfaced together: when he went to make sure the guy understood they were a buddy pair he realized this guy (the only person that day not on the rental gear) didn't have an octo and the extra time underwater wasn't worth it. Without a doubt, in a group still of 5, including the DM, he would have been fine. But as a 'buddy' he wasn't interested.
My alt air is on my inflator hose. My primary second is not extra long. My diving is open ocean up to 60 foot drift dives no mandatory deco. I also carry a 6cft pony. I want to CESA with something to breath. I could hand off the pony if needed but at 60 foot no deco you are 60 seconds to 120 seconds to the surface.