What do you mean? Where updating your Twitter would update your FriendFeed, which would update your Twitter, which would update your FriendFeed, which would update your Twitter, which would update your FriendFeed… ? That’s not how it’s designed though. In any case, it’s up to the software developers to ensure that this can’t happen, and from my working with the Twitter API, it’s clear that they have a heap of abuse limiting in place to ensure that a flurry of the same update couldn’t happen.