With a single AL80 you might use about 5 pounds of air (63 cuft) during the dive; with two AL80 you might use twice that, but probably less (not likely to double your dive time!). So you want to be 5+5=10 pounds heavy at the beginning of the dive, so as to be neutral at the end. If 12 pounds is right for you with a single AL80, that's like 12=7+5 for air to be used; with two AL 80 it will be 17=7+5+5 for the air to be used. You are not using the pony now; ii it is dead weight, maybe 1 pounds full wirh a reg. So without the pony you need 18 pounds for your sidemount to start....adjust from there.....