I agree with you.  I cannot think of any reason why someone would want to have co-trustees hold title in survivorship.  And, when the issue was presented to me, I advised against it.  The trust document is sufficient and survivorship is not necessary, in my opinion. 

I only asked the question, because I wasn't sure if it was even possible to do it.  And, I started to wonder what would happen if someone did take title that way.