Could something be done with the steam cloud services where the saved files are stored on the steam cloud and the latest one is used when the locked coop game is started, and then if the game is running while someone tries to start their solo session, it connects to theirs instead? alternatively storing the shared saves run in solo could act more like a .diff file like a github push, where it stores what was changed this session, and then those would get reconciled on a merge when possible, so if each player just controls their characters, the saves are fine, unless they both took the same item and tried to sell it or something.
