Old standards allowed 5', but 7' is the normal configuration. It allows your buddy to lead you out of a cave, through narrow openings, while breathing off your air. The hoses and guages must be routed to avoid problems sharing air. An air sharing or 'S-Drill' is conducted repeatedly during the course to verify no problems exist in deploying the backup reg. DIN is strongly recommended.
Yokes are easier to knock off, and they blow o-rings with little or no warning.
No cavern diving at night is allowed. Cave diving requires three lights. The Cavern Class itself. It is a two day course. Most of day one is academic and dry land exercises.