Not sure where the value of $800 is coming g from, but I don't see a Williams card like that being more than around $500 on average.
Dual signed might sell at the high end of the range to someone who is collecting that set. They will pay a bit more for the dual signatures to save time and risk sending it for the other person to sign. If someone just wants a signed card of Williams, they might play less for the dual signed. The second auto might detract from Ted in their mind.
Here are a couple examples that I used for my analysis.
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