You need to have a grounding electrode and grounding electrode conductor for each buildings' service entrance. You do not need to bring the grounding condoctor from the transformer to the buildings. When your four wire service enters each building's service entrance equipment, the neutral and ground busses must be bonded at that point. These in turn will have a system grounding electrode conductor to your grounding electrode, be that a Ufer ground, ground rods, building steel, etc..., with the appropriate bonding of water, gas metal service piping, etc....
If as a later question asked, you have exterior service entrance equipment adjacent to the transformer, both the transformer star-point grounding electrode conductor and equipment grounding conductor should be tied to the same grounding electrode system as the service to prevent differences in potentials, circulating ground currents, etc....
I think the above agrees with previous responses for the most part. We see this on many campus type distribution systems where the owner is 'the utility' for each building. Are you saying that both of the building's individual meters are located at the transformer?
Regards,
EEJaime