I recently worked on a 767-300 that was not closing the DC Bus Tie Relay when one of the T/R's failed.
The system uses a DC Tie Control Unit that senses a lack of 28VDC on either the left or right DC bus. The unit will wait 12 seconds before closing the tie relay.
The control unit is located on the P6 panel (forward section).
A simple test after replacement is to pull a T/R breaker and monitor the DC EICAS screen to verify both sides still have power.
A simple depiction of the system.