Yea, soooooo, I have been philosiphying (is that even a word) over wan, and I'm going to try it, tomorrow, but flex is my girl, I whisper sweet nothing to her nightly after I learned she is now a pariah. I can't even begin to describe my broken heart. But I'm hoping wan surprises me.
So currently I recommend staying on flex for the moment, because currently I have no real way to help you with wan. Somebody from rachio will need to chime in here. I have no experience with it, and I can't back it up with any of my own research and trials (half assed scientific, I only have so many $). I'm really underwhelmed by a single zone kicking off all zones in the schedule. I mean this is in and of itself, not an issue, I can fully justify that behavior...it's the fact that multiple schedules do not thread together at all. If we had that ability, then flex=wan.
I'm not saying wan is bad, don't take that from this, I'm just assuming you have 6+ zones and you, like me, would want to begin watering around 4:30-5 am.
Now, where your zones overlap, i don't think I worry too much about disease with the increase frequency unless you have had disease issues in the past, have you?
I think I gather that one zone is shade and the adjacent is sunny, and I'm assuming you have rotors in these zones. If disease is an issue, I would turn the screw that is on the front of the rotors to reduce the distance of the border heads in both zones, to decrease this overlap...this in turn will decrease your head to head coverage for these head, in turn bringing down your zone's efficiency a little but it is better to throw extra water to recover than it is to drop $80 on some fungicide and treat for a month. Advantages are less disease if this has been an issue, and the grass in this zone should attempt a deeper root depth (if on a slope then that is awesome as water will move down that slope between waterings providing incentive for root depth). Disadvantage is that this border could become less drought tolerant under certain circumstances which are too many to enumerate.
That is my $0.02 not having seen any part of the landscape.
Some more,detail would be nice, now long do your turf zones run, how many turf zones, what type of turf, what type of heads, what type of soil, what agriculture zone are you in?
Do you use growth regulator, do you use a foliar fertilizer, in soil do you use slow release or urea/ammonium nitrate?