Dependence of EMF of a cell on stoichiometry
The electromotive force of a redox reaction is not dependent on the coefficients in the balanced equation as it is an intensive property (i.e. a property that does not depend on the size of the system). Thus, your reasoning is incorrect. 

You can read more about electrochemistry here: https://chem.libretexts.org/Core/Analytical_Chemistry/Electrochemistry/Basics_of_Electrochemistry

Also, I am wondering if your textbook is actually correct because $\ce{Ag2+ + e- -&gt; 2Ag}$ and $\ce{Ag+ + e- -&gt; Ag}$ are definitely different. Thus, the standard reduction potential of silver (I) should not be used. Instead, the standard reduction potential of $\ce{Ag2+}$ should be used. Someone who is more knowledgeable in this area please correct me if I am wrong.