In most of Europe the going rate for AT&T is $1.50/minute, so if you call each other that will be $3.00/minute (a minute on each end). If you're really just planning on this as an emergency back-up, that's ten minutes of calls before the roaming plan makes sense. Another option is texting each other - text while roaming is something like 35 cents each. OCTeckels and I do that to coordinate when we get split up.

I know with the AT&T data roaming plans, you can backdate the start of the plan to the start of your billing period. It's possible that you can do the same with the voice roaming. You can manage this yourself on the AT&T website, and you can go all the way to picking dates and still cancel if you want to check in advance.