My wife and I are in the same boat as you and your wife. We are also planning to hike the PCT in 2015.
I think you are right to consider the possibility that you both might not be able to hike together the whole way. Injuries can easily cause this to happen, let alone all the other potential reasons...
I think you first need to take a step back and ask yourself if there is any possibility that you would both be hiking the trail simultaneously, but not together. This situation, in terms of gear considerations, is a little different than one person hiking while the other recovers from an ailment in town.
I know that my wife and I have no interest in hiking apart, so if we're both on the trail, then we are going to be together. This means that we will only ever need shared gear that we can also use solo, if need be. No duplicates are necessary for us.
If you plan to hike apart, then you would both obviously need shelters. You'll have to figure this out beforehand to plan accordingly.
If I was hiking any significant portion of the PCT alone, then I would strongly consider carrying a Six Moon Designs Gatewood Cape poncho/shelter. I'd probably bring the Serenity Net Tent as well to clip in under the Gatewood Cape.
If you and your wife will always be hiking together (while you are both hiking), then I think the ZPacks Duplex makes a lot of sense. It's basically the lightest fully enclosed two-person shelter out there and is certainly light enough that carrying it solo is not a huge deal (and it would be a palace for one!).
I have never used any of the ZPacks shelters, so I can't comment on the durability of 0.51oz cuben vs 0.74oz cuben.
My wife and I unfortunately don't have the money for a $600 shelter right now [sigh], so we will be using our beloved, but much heavier, TarpTent Double Rainbow.
On a more personal note, I think you should ask yourself: do I really want to hike apart from my wife? Your marriage will be lasting much longer (hopefully) than your thru hike. Which do you want to prioritize?
Just carefully consider the ramifications on your relationship of not sharing the same PCT experience together. These hikes are usually life changing events, and I for one would like the changes to happen to both my wife and I together so that we can share that experience as partners and let it serve as a bond, not a wedge.
Not everyone will agree with this mentality, and that's totally ok, but it's certainly something to think about...
In my mind, one spouse hiking on while the other recovers in town from an ailment is one thing, but willfully hiking apart while you both could otherwise hike together is another thing entirely.
I say this all without any judgement though. Every relationship is a little different, and what works for some might not work for others.
Just be sure to tread lightly my friend, and good luck. Maybe my wife and I will see you guys out there! :)