Just have two character sheets. Share equipment and at least the same constitution score so you can share hit points. It’s a few extra hit points than the Warlock should have but also it’s a pretty unique scenario. As long as they aren’t trying to cheese the system it would work fine.
Yeah, either give both the level or—IMO the better option—the GM can just give them level-ups twice as often. More work for the GM of course.
Just have two character sheets. Share equipment and at least the same constitution score so you can share hit points. It’s a few extra hit points than the Warlock should have but also it’s a pretty unique scenario. As long as they aren’t trying to cheese the system it would work fine.