Totally doable and super easy, you’re overthinking it 
If you repeat the auth process, i.e. how you added your first character, but this time select a different character, it will also authorize that character. You then will be able to specify which character should be used for a function (which defaults to whatever character you set your MAIN_CHARACTER variable to), by doing like characters_character_mining("Blacksmoke16").
If you need to add another character that is on a different account; click Cancel when you are at the part where you need to select a character, login to other account, and select the character you want to.
Checkout this post in regards to how to best manage authing other accounts you do not own based on your situation.