One would fill the Autumn side with Xavier and Magneto. Fulfilling the traditional and historic side of mutantdom.
One will fill the Summer side with Nightcrawler and Storm, fulfilling the empathetic X-Men.
Xavier and Magneto asked Scott to join and Jean to come back, they both declined.
Namor has turned down the offer as well.
Keep in mind it seems like whoever fills the last two seats, it looks like Magneto and Charles have to agree on them.