These are just two conventions for writing the EMF formula but both work.
If you write the two equations (half-reactions) so that one is undergoing reduction and one is undergoing oxidation, and you change the value of E by inverting the sign, then you can just add the electric potentials.
** if reactions are written so that they are undergoing reduction (convention), you would flip the reaction [so it will undergo oxidation from L-->R] AND flip the sign.
If you identify two half-reactions but DO NOT flip the reaction/sign, then you need to use EMF = Ecat -Eanod.
