In the gliding world we have something called an "annual insurance share"
This is where someone agrees to pay a portion of the running costs for the year in return for the same proportion of usage. That person would also agree to any insurance excess or repairs accrued whilst they were using it. The full title of the glider would remain with the owner
I am not sure if this would translate to powered aircraft. The advantages to the owner are reduced costs. To the other person, no capital cost unlike a syndicated aircraft.