I had a pair of 10L Steel tanks (low pressure light weight) that I had attached to the Scubapro Night hawk. This only has 20Kg or 45 lb (something like that). I also used to carry a 4 liter aluminum for deco gas, on the same rig. I never had problems with that. It was light and manageable.
Now I'm using twin 16 liters, and they are back busters.
I use a 100lb lift OMS wing for that. This comes handy especially when I have the stages attached (12L / 180 bars (lighter than normal 12's)). Even in the water I feel my back arching. If you want redundancy and lots of gas you have to pay for it some how.
Make sure you have another way of inflation if you rig up the twin 15 L. Use the dry suit or a 2nd bladder.
Lawrence D