I would recommend a designated field (Sendto) which will store your addesses which you will send to.
Then you can have conditional formulas to handle either scenario.
Let's say you have an Email1 dropdown and an Email2 dropdown.
Email1 has a rule that if Email2 is blank it sets SendTo to Email1. It also has a rule that if Email2 is not blank then concatenate the 2 with a ";" in the middle in SendTo
If they pick Email2 it has the corresponding logic.
Then you have handled the either or scenario.