How to calculate the electrical potential and mols of electrons transferred from combined half cell equations?
TL;DR

This basically boils down to the fact that $\mathrm{E_{cell}}$ is an intensive property, not an extensive property. 

Long Version

Lets define some terms first:

Intensive Property:
A property that doesn't change when the size of the sample changes. For example, density and concentration are examples of intensive properties

Extensive Property:
A property that changes when the size of the sample changes. For example, mass and volume are examples of extensive properties

$\mathrm{E_{cell}}$ is the energy in joules of 1 coulomb of charge (1 volt = 1 joule per coulomb). Hence it is an intensive property. Therefore the number of coulombs of charge or number of electrons doesn't effect the value of $\mathrm{E_{cell}}$. 

Another way of thinking of it is that by doubling the amount of moles, the  amount of coulombs and joules also double, hence $\mathrm{E_{cell}}$ remains the same.