Where he said; guard for me today, etc. - דאמר שמור לי היום כולי
Summary:
If the owner commits himself verbally to be in the employ of the שומר immediately it is considered שמירה בבעלים, but not if his commitment will begin after the שמירה. It is more reasonable that שמור לי ואשמור לך is considered ש"ש where they are not partners than when they are guarding items which are owned in partnership.