I did also dive this way in some occasion. Just a single second stage, no alternate source.Hmmm. If the diver is running both a primary 2nd stage and a secondary 2nd stage on his single 1st stage, then doesn't he/she have an "octopus" regulator?
Maybe a reg requires more hoses (e.g., a LP BC power inflator hose and a HP SPG hose) to be considered an "octopus" reg. (I really don't know...)
rx7diver
For shallow diving well within NDL, with a reliable buddy, it is still reasonably safe: at any minor inconvenience you just ascend.
From less than 10m depth, in warm clear water, it is simple.