If you have one regular character and one retro character, what would the 2P category would that fall under?

Thank you for your time and answer.

Retro characters are separated from the Main cast because of gameplay differences. If your co-op run includes one of them, post it under the Retro category.

It's as FireRain says;

Regular/Regular = Regular

Regular/Retro = Retro

Retro/Retro = Retro

So you would submit it under Retro 2P

Greatly appreciated.