They've already done them for new planes. Both Speed vs. Cunning and Heroes vs. Monsters.Won't work for new planes very well. They can now do two pw dds and I think they will.
In the past they used the fall duel deck to build hype (with preview cards) and the spring duel deck to stabilize sales a bit. I guess it ultimately comes down to which is more important.
I am curious to see how the second block release schedule shakes out. Typically the 3rd set isn't around for long before the summer supplemental set hits. It's going to be weird to draft Tears for a few weeks, then draft [Conspiracy 2]* for a few weeks, then draft Fears/Tears.