NonFlushingLav,
I never saw a TCAS unit installed that talked to the ground prox
Time to look again; commercial installations have priority logic, probably in the alerting system. In very unusual and difficult circumstances the pilot is not confronted with conflicting messages and thus does not have to make a decision.
CS 25 AMC 25.1322 Alerting Systems
6.7 There should be only one aural signal at a time. If the possibility of two or more aural signals at the same time cannot be avoided it should be shown that each signal is clearly intelligible to the crew. The order in which the signals are presented should be that in which crew action is required.
Re SOPs, often there is need for considered debate about SOPs, the good ones, poor ones, personal SOPs, and those which are rules. ACAS SOPs are rules with very few exceptions.