If you attached the additional support snugly without releasing the built-in stresses in the original support structure, then the state of the bearing of the new support will depend on the load and the deformation that come after the addition. Note that the stresses in the original support will continue to increase, albeit at a reduced rate.
However, the entire load on the beam, including self-weight, will be shared equally by both the original and the additional supports, if the stress in the original support structure is fully released by "jacking" and "repositioning" operations prior to and after the addition is in place.
Another requirement to ascertain the proper transfer of loads from the existing structural element to the new element is to develop the composite action between the two elements through adding connectors/shear studs and careful grouting at the interface. Without such, while beam-column joint can be fully in contact, separation could occur in between the existing structure and the additional beam element, as they literally act independently from each other, thus the state of stress at the interface is highly uncertain.