We've discussed this in depth, and we have arrangements. Done right at the beginning, when I saw all these sad pets at the shelters whose parents had died, and vowed it would never happen to our furrballs.
In the event both of us pass simultaneously, my sister will take them all. They are all written into our wills, with a trust fund set up to take care of them for the rest of their lives, and my sister as the executor. We have alternate contingency plans also.
She is intimately knowledgeable of the level of care, attention and affection we lavish on them, so we feel comfortable that she will take very good care of them.