The value of a multiple signed baseball cannot be priced by the value of each signature added up. Multi signed baseballs are valued as a group of signatures, and how desired that group is as a whole. This does not necessarily mean that two signatures will sell for less than one, but in most cases it does, or at least not the full value of the two signatures added up.
Value is based on past auctions and sales. I could not find a Kirby Puckett Harmon Killebrew dual signed baseball that sold in auction to help you with an accurate value. I would roughly estimate the value at about $100.-$150.
Value will vary based on the condition of the ball, grade of the signatures, ink used, placement & display of the signatures (eye appeal) and the type of authenticity that accompanies the ball. Signatures that are not properly authenticated could sell at half the market value or less. -Steven
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