As far as which two seater you choose to fly, that depends on whether you want a bomber or an aircraft which can carry bombs but also fight (occasionally). This would also then determine your tactics

So for instance I enjoy flying the Roland CII. It's not overly manoeuvrable but is relatively quick by 1916 standards. So you can catch a DH2 up if you want, but that's flying a two seater 'aggressively'. The same goes for the Sopwith Strutter.

But your question was about defensive tactics. Most ( if not all) two seaters don't climb as well as single seaters, so my tactic of choice if I'm in trouble is to get down towards the ground as quickly as possible and 'tree hop'. This takes climbing ability out of the equation, if the enemy fighter wants to carry on attacking you, he'll have to do it from behind and can't get beneath you. Giving your gunner the opportunity to hit him. It makes the fight two dimensional, rather than three.

Naturally you have to be wary of where you are when you are flying low, avoiding enemy airfields and camps etc, but generally I've found this tactic works well.

Edit - I should point out this works with rear facing gunners best (although I've done it in Aviatiks, Caudrons and FE2b's too).