I played a campaign once where I had a gestalt character. Meaning I had 2 classes and leveled them both up simultaneously, but I don't know of a way to make a character sheet on dndbeyond that functions like this. Any tips?

This site largely sticks to the rulings / game as given in the books where levelling two classes at the same time is not permitted - by the rules you only choose one class to increase each time you level up.

This site cannot accommodate homebrew reworkings of the core game mechanics.

Best way to do it, is to make 2 character sheets, one for each class.

Not pretty or elegant, but still works.

Same race, character, but 2 different sheets for the classes.

You can also just make homebrew feat(s) that represent the other class. Maybe have a feat for each level and every time you level up you apply the next one to your character.

Fair enough, thank you all for your insight :)