I would have to see the signature in question. I have never seen a signature where part of the name was signed on the sweet spot and the rest was signed on a different panel. I HAVE seen balls where the signature was signed in a straight line, as most are, and the signature was not CENTERED in the sweet spot. They either started too far to the left, or they started too far in and the signature extended out to the right.
That is still considered on the sweet spot, but attractiveness of a signature is a major factor in value. For the same reason a valuable baseball card in otherwise Mint condition but is very poorly-cemtered will sell for substantially less than one that is properly centered. Eye appeal is everything when it comes to value. A genuine Babe Ruth ball with the signature off to one side or the other will sell for thousands less than one that is dead-on in the sweet spot (and I've never seen one - Babe signed a lot of balls and players knew what the sweet spot was a hundred years ago).
Now, if some stacked their name - like this:
...with one on the sweet spot and the other on the panel above or below, that would have a major impact on value. I've never seen that on a Mantle ball, either, and I can't recall seeing a stacked signature on a ball, ever. I see it on paper items occasionally.
I bring up stacking only because I'm trying to picture something where the whole signature wouldn't be on the sweet spot. If you have what you describe, post a picture of it.
Hobbyist going on 30 years, buying and selling for 25.