[Lordstan]

Jeff,
I think you have 2 separate issues.
Shelly answered the first. By definition, a single signed ball is signed by only one person.
The second part is what is "acceptable". That is a personal decision and I don't think there is one right answer. It depends on multiple variables, including how does it display, how nice or meaningful is the most important sig, how much is the sale price, and how much does the buyer have to spend.
For example, If I found a ball, in my price range, signed on a side panel, "To Mark, my friend, Best Wishes, Lou Gehrig", I wouldn't care who else is on the ball. It still would not be considered a single signed ball, but that wouldn't matter to me.

Also, I don't think extra writing like a date or a description by a third party would change it to a multi signed ball, but it might change how desirable the ball is to an individual collector, either better or worse.
Beauty is in the eye of the beholder.

Mark