side-slung would make more sense to me.
1. on the back your rig is going to be very unbalanced when kitting up and moving around.
2. with side-slung you can kit up with the 15l then clip the 7l on, will make kitting up much more pleasant. same for dekitting, especially into a RHIB. you can carry the tanks to the boat separately too.
3. 7l's are lovely side-mounted, and they don't get in the way at all as you might expect. you can pretty much forget you are wearing one. (sorry, only applies to ali ones i suppose! my mistake)
4. you can easily route the hose to a necklaced reg if you want quick access to bailout gas.
5. cheaper to rig up a stage-mounting than to buy a strong pony clamp! (make sure the bolt snaps have huge eyes to get your thumb through.)
__________________
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
—Robert Frost
k