Why do ID proponents assert the existence of only one Designer? If you were to accept ID, would you not have to accept the possibility of multiple Designers?
Logically, knowing the mistakes that we humans can make (and do make every day, however good we are at what we do), the answer would be "Yes".
But God is not human; if (and I say "if" for your sake), he did create the ability for life to exist on Earth, there would have been no need for more than he, because hof his "perfect" abilities.
We, as humans, can have no comprehension of what God's abilities can actually be like; our minds have not the capability of understanding the mind of the greatest of scientists like God, whose powers are so much greater than anything we could imagine.