IMO, I would grab the tandem especially since Allen is taking the ice. You'd lock the Anaheim goalies, eliminate the headaches of having to worry about the St. Louis/Anaheim goalie splits & Anaheim may have to play hungrier down the stretch to make the playoffs.
Besides last night's shellacking by Pittsburgh Anaheim has been playing decent hockey recently. I would grab Andersen. Even if Andersen and Gibson split the rest of the way you should get more starts than keeping Elliott and Gibson alone.
The problem with owning half of a tandem is if the other half gets hot you could get screwed. Owning two tandems could happen with both guys. It's not like Allen and Andersen are slouches. I'd go with Andersen to guarantee you're not going to get screwed over. And like havana said, if Andersen gets traded then you've still got two starters.