Voltage applied when charging a galvanic cell
Voltage is how much push is needed to move the electron, and is the same whether for one or one trillion electrons. Electric charge, measured in Coulombs, is what changes when more electrons are moved.

Your reasoning that moving twice the charge doubles something is correct: instead of ~1.6 × 10−19 Coulomb for one charge, it's twice that, ~3.2 × 10−19 Coulomb for two of them (no discount for large charges, but I hope this kept your interest).