Iron City
You seem to be saying that the ground radar receives both SSR responses - one for the radar and one for the TCAS. The ground system then 'resolves' the dual returns by computing which one is the most consistent with the previous accepted return with the same squawk.
If this is so, it begs the question how the TCAS distinguishes the two returns. Presumably it carries out a similar guess computation although it is going to be more tricky as the TCAS is closer to the transponder and thus must cope with more rapid changes of angle and distance/time delay?
Note I am assuming TCAS simply interrogates transponders ie the only thing special is the TCAS receiver. There is no TCAS-to-TCAS dialogue.