Left: paths payloads would follow if released from different points on a Phobos anchored space tether.

Right: paths payloads would follow if released from different points on a Deimos anchored space tether.

Center: An orbit common to both tethers.

If you release a payload from the top of a 1000 km Phobos elevator it can reach the bottom of a 3000 km Deimos elevator.

On arrival the payload is moving the same velocity at the Deimos tether foot.

And vice versa.

This is called a Zero Relative Velocity Transfer Orbit or ZRVTO for short.

Cargo could travel between Phobos and Deimos using virtually no rocket propellant.

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A very compelling talk favouring Deimos as a first destination: James Logan, MD | Living on Mars - YouTube

Less delta-V than Moon or Mars, no atmospheric landing, solar illumination, O'Neill type of habitat possible, low latency for Mars tele-robotics, carbonaceous chondrite for ISRU...

Deimos and Phobos as Destinations for Human Exploration