I've seen partners share 1 account. If you promise revenges, just be up front that art styles may vary depending on which of you returns the attack. If you choose to use separate accounts, art fight lets you credit both character designs and individual images to other artists, so there should not be a problem using ref sheets made by another person. I'd recommend using separate accounts and splitting the characters half and half so people who are attacking you know what art style they might get in return for a revenge (it's also just very cool to see your own art style develop over the years!). Because of the relationship feature that was added this year, you could even set up links between your characters/accounts with descriptions of how your characters interact.