No. Intelligent Design would not be disproven by a second planet with living creatures. There's no reason why a Great Designer wouldn't place life elsewhere in the universe.
But a second planet with living creatures would place many Biblical principles in doubt, such as the one that called mankind God's special creation. This and a couple other references compel Christians, which I count myself as one, to believe that there cannot be sentient life elsewhere in the universe. It would shake to the core the foundation of Christianity.
That's another reason why scientist are so adamant about funding SETI (Search for Extraterrestrial Intelligence). They want to disprove the Bible.
Exactly my point. The uniqueness (or lack of same) of Earth has nothing to do with ID's validity, unlike what the author contends.