first order of business would be to fly each two-seater in free flight in the observer seat. then sweep the gun through it's entire range of motion (up and down, side to side). get this burned into your memory. your AI observer can only open fire if he can actually point his gun at the enemy. if you're in a fight, your job as the pilot is to either run away and/or fly in such a way as to allow your observer to fight back. I find that it is generally best to fly as the pilot. there are a handful of machines where you could plausibly fly as just an observer.... but that's probably only the FE2b and the Caudron G.IV. that's because they both fairly early war planes that have a bow and stern machine gun. (you could probably do this with the Bristol Fighter F2b as well)
Polovski is right. you activate the full auto-pilot with CTRL+Shift+A in order to jump into the observer seat and fight off pursuers. the default is that the aircraft essentially goes 'dead stick' once you leave the cockpit. with some machines, like the BE2c this is perfectly acceptable - as it was a very stable flying machine. in other machines, like the Sopwith Strutter or the Caudron, it's likely to start listing over to one side.
how often you do switch from full AP to manual control will vary from one plane to another. it also depends on how aggressive your observer is. if you have an experienced AI observer you might be better off just flying the machine and letting your AI observer shoot up the enemy.
that being said, even pretty good observers will open fire at ranges that are simply too far. so it's not uncommon for me to set up a pursuit path and fly auto-level if I know I have a faster two-seater. by flying auto-level and taking up the observer station I can frequently sneak below and behind enemy aircraft and shoot them up from the observer's station in the nose of my Caudron G.IV. it requires a lot of careful planning and patience, though. and you are not at all guaranteed to shoot them down. even if you DO shoot them down you will stand a pretty good chance of not getting the claim confirmed if you lose wingmen in the process. although I haven't become a confirmed ace in the Caudron yet with any one particular pilot I've managed to shoot down several dozen German aircraft using this type.
I think an 'observer' career is possible... but I agree that it can't really be done with the Roland C.II - that's a plane where you want to fly it as a pilot far more often than not. besides, it has a fixed forward firing machine gun.... so as long as you manage your speed and altitude properly and pick your battles very carefully.... you can treat it like a fighter.