Since you mention that you only want to use your iPhone's for voice calls, there is another solution:

Use your iPhone's only over wi-fi (Skype) when in a hotel room or hot spot.

Purchase 2 cheap pay-as-you-go phones once you land in Europe.

This way, you use inexpensive phone service provided by a Europe carrier for voice calls to each other and only use the iPhones where you can get free wi-fi and avoid all ATT charges for international use. Google "pay as you go phone Europe" and you'll see several choices, the easiest thing to do is grab them at a 7-11 at the airport.

BJ