Auctions usually. Have people bid for slots individually.

Hmm, that sounds good.

Depends on how many are in the scene. If it's just a two-person scene, I've seen it offered as a combo package, and separate offers for each slot. For larger scenes, I've seen it done usually as having each character slot be a separate auction.

I would say it depends on the number of characters. For large scenes, artists usually let people bid (or have a set price) for individual slots.

If it's 2 or 3 characters only tho some often propose an option to get bot slots at once.
As a commissioner, I am more inclined to get both slots at once cause I would be anxious about who gets the other slot. But that's just me.

Multi-Character usually has slots for each of the characters with different prices (unless you want all of the slots to be similarly priced), commissioners choose which slots they want their character in.

You could raffle off slots between all the people interested as an example. I have seen that. Some also just go first come-first serve.