Not that I'm recommending doing this, but I just want to put it out there as having been done...
First, what kind of exposure suit are you wearing?
This will have an effect on your ballast/lift situation
Last week in the pool, I ended up with a hodge podge of equip to use (all resulting because I forgot to bring a DIN/Yoke adapter for my regs...).
I happened to have my doubles regs and my SS backplate, and there happened to be a set of double Al80's with DIN manifold... only problem was that the only wing I had was an 18# pioneer...
Well, with a 5mm wetsuit, the SS backplate, and no additional weight, the 18#'s of lift was enough for me to keep me at the surface when I wanted to be, let me descend when I wanted to, and keep me neutral at the 13' bottom.
(Although, the position of the OPV on the singles wing doesn't fit nicely with doubles... I kinda had to 'convince' it to get on... )
anyway, the point... I'm sure anything in the 40# range will be more than enough for double Al80's.