Last tour I went on with the fiancee, one of her tires needed topping off every morning. The other three tires (between the two of us) were fine for most of the week and a half without being touched. No puncture, all four tubes came from the same place, same brand, same model. Never been able to figure it out. I've got the same tubes in yet another bike, that I have to keep topped off fairly regularly.

I've just come to expect some individual tubes do a better job than others, with no real correlation to brand/model/price. I'd bet if you moved the tubes to the back on your bikes, you'd see the issue there, it is just coincidence that they both happened to be front.