Cristina imagines the two routes her life can take based on one decision she makes. Meanwhile one of the hospital’s patients decides whether or not to live.
What is that "one decision"? While we don't know for sure, the promo suggests it's the choice to be with Owen, for real, vs. the choice to finally let him go. Meanwhile, we know that pivotal patient will be played by Mad Men’s Charlie Hofheimer, and he also has a wife, played by Elise Castor.
The synopsis uses the word "meanwhile," but it sounds like that patient trying to decide his fate actually has an effect on Cristina's mindset as she thinks about her possible futures.