The camstraps are a breeze - especially when wet. I don't have one sitting here in front of me at the moment, but if you unthread the last tab, and use the tail of the camstrap as a lever, you can pull the strap tight on the tank very easily - the webbing simply slides through the buckle slots. Rethread that last slot, and close the cam buckle. Use the velcro to hold down the excess tail. It's not a buckle design unique to the O-Pac.
Re doubles - the camstraps simply unthread from the O-Pac - they're not permanently attached. There are mounting holes for doubles, but I've never attempted mounting them myself - I think Patrick Duffy has an accessory in the works for easily mounting lightweight doubles to the O-Pac. I haven't seen the new accessory, just aware that it's a work in progress - I don't know what size tanks it can handle up to.
