No matter what your octo is, by far the thing that makes air-sharing easiest is a nice long hose, so it sounds like putting your G250V on a 5 ft hose, then getting either a metal case 109 or an older G250 (or G200B) on ebay as a bungied alternate would be perfect.
The 109s make fantastic alternates; they're relatively cheap ($50 or so) amazingly easy to rebuild, they breathe extremely well, and they're bulletproof including being tolerant of getting tanks dropped on them. (don't ask how I know this)
Plus they're shiny metal, so they provide a good target around your neck for barracuda attacks.