Hi Fill,
Yes, two-seater crews were issued with the Heinecke - after all, their jobs were more important than those of the Jasta fliers. I assume the observer used the seat pack just the same as the pilots did. I've never read any contemporary information about this (this is a job for Rammjaeger!) but it was very likely procedure for the observer to exit the aircraft first, and may even been easier since he didn't have to control the airplane at the same time. Here is a crew of an LVG C.VI getting ready for a mission (maybe) and both wear the harness.
I'm sure others will have more opinions and info and correct my errors. The only accounts I've seen or read of actual parachute jumps were by fighter pilots (Udet, Baeumer, Praclik, Raesch, etc. - and accounts of failed jumps by Loewenhardt, Friedrichs, and others) but I seem to recall somewhere an account by a British pilot who shot down a two-seater and saw the crew take to chutes.
Greg