Lets say the Duke of Saxony somehow inherited the County of the Palatinate, would the Duke of Saxony then have the voting power of two electos, or would a new elector have been chosen?
In 1777 the Wittelsbach electorates of Palatinate and Bavaria were merged when the junior branch reigning in Bavaria died out. The senior branch still only retained a single vote and no new elector was appointed.
This is opposite to how voting rights in the Reichstag were determined as every territory was assigned a (partial) vote and whoever was the rightful ruler of the territory was able to use that vote.