A portaledge is probably the single most expensive item of gear or you'll ever buy for climbing. So, you want to be sure you get the right one for you. Double or single? There are arguments for both approaches, but consider:
Double portaledge benefits
a double weighs a lot less then two singles - maybe not a big deal when you are hauling, but you'll definitely notice it on the hike out. (Metolius Bomb Shelter example: about 11 lbs. for a single compared to about 13 lbs. for a double)
a double is less $$$ than two singles
a double usually is faster to set up then two singles
a double can be easier to set up at a cramped bivy; while two singles often requires some crafty placement, with one of them sometimes in a Less Than Ideal (LTI) position or off of a marginal anchor, or both
Even for the solo climber, a double offers you more room inside to put your gear, lounge on, and hunker down in in the event of a storm.
(Downside in a storm: the doubles can become a larger sail, potentially tossing you around.)
Why might you choose a single portaledge?
If you are fairly small, a single is easier to set up, or you're fairly tall, giving you room to sleep diagonally.
Singles typically get more “square footage of bed space” per person
If you know you have stinky feet or snore a lot or have some, show we say, intestinal issues, having a little distance and privacy and your partner might be a good thing. Ergo, single.
Recommended double portaledge: Metolius Bomb Shelter or John Middendorf's D4 ledge, available at bigwallgear.com