This is the thing that has always bothered me about dual displays. If for example a Kiss display floods then there is saltwater across the cable ends from the cell, electrically this is the same as saltwater across the cell pins (ignoring the tiny resistance of the cable), a second system attached to the same cell is unlikely to be unaffected. Uri's HUD system has some protective circuitry in the head to solve the problem if the HUD cable floods (so it doesn't affect the original dislay), but the problem on the Kiss display side remains.
I did look at the Shearwater but this is one of the things that put me off. All 3 cells are connected by a single fischer which is protected by a single O-ring seal, and I'm concerned they might all be shorted at once should this fail. I asked Bruce about this at the NEC dive show and he reckoned this O-ring doesn't fail, but it is potentially a single failure point which could compromise all your PO2 monitoring on both displays. The hard-wired version goes some way to removing this of course, but then you can't use it OC.
In the end its all down to risk-benefit analysis I guess.

Neil