I have .685 on my Brass (KP2 like) pirate and I have not really had any issue with that yet. It does have a cocker detent to hold in a ball.
I have been shooting almost everything out of a .675 brass freak insert on my other guns. I shoot Valken Redemption and Graffiti 99% of the time which sized around .682?
If you intend to double pump on a regular basis, I would recommend going with the .685 but I would also make sure you get some kind of detent system installed. Either the palmers ones, Widgets or whatever...
Getting 60 shots off 2 12gs in the config you want might be a stretch...but might be possible. 20 each side might be a more reasonable estimate and if you get more it is a bonus.