My thoughts on this have always been to bring them both from the start. If they are supposed to be the stars of a Green Lantern team-up movie, you don't want to favor one over the other beforehand. WB better not make the mistake of having a "protagonist" Green Lantern in the DCEU who shows up in a Justice League movie first and then everyone else is his supporting cast in GLC. It won't work, if it's supposed to be a team-up movie, treat it accordingly.
I'm thinking either Jessica or Kyle. Guy and Simon would probably get the shaft at first.