Shared Backyard Gate

[Zee Ayezain]

Hi there, 

I have a question on a row house property with other properties on both sides of it. Each 2 properties share the back stairs from the back yard that lead to the third floor. Therefore, the backyard space doesnt have a physical barrier in between but clearly each property owner owns their side of the backyard. There is a shared backyard wooden door/gate, can we put a lock on it? Who would pay for it? Should it be a combo lock? this is a frequently used door by the tenants and if its keyed then both sides need to have a key or if it has a combination then we need to share that with the neighbors. Any recommendations on the best kind of lock or solution for that?

Thanks a lot!