As many already said you won't find a wing good for doubles and singles, for singles 30 lbs is more than enough, specially with AL80 in salt water, for doubles it depends on your configuration, other equipment and so on, but for most 40 to 45 lbs will do.
The reasons why 2 wings I could point out as...
1 - Using a 30 lbs with doubles the problem is pretty obvious, unless it is a AL80s double, it won't give you enough lift.
2 - Using doubles wing with singles, I see two problems, which are in fact conected, first, with a wing too big there is two much of the wing free, generating more drag, and in overhead invironment, specially in wrecks, can make a mess. The other is a consequense of the first, when you inflate the air tends to get trapped in the upper curve and it is dificult to vent, causing a lot of trouble to control ascent.
My configuration is 30 lbs for singles, 40 lbs for small doubles (up to steel 95s more or less) in salt water and 60 lbs for heavy doubles, heavy gear, specially in fresh water.