How many of those previous 8 games came since Demko's return from a long absence due to injury?
I think teams make mistakes of this sort all the time. Always give the start to the guy who got a shutout in the last game, even if it's back to back. Ride a guy who's winning. Look at his record against the team coming up. Who cares if the starter gets tired and the back-up gets cold?
Demko and Lankinen both need rest at the right time, and they both need to get in a rhythm. It should be something like a 60%Lankinen/40%Demko split the rest of the way. Avoid playing Demko twice in three days.