Airbus TCAS
1-) This is not named as one of "Golden Rules" but "Memory Items" in Airbus terminology.
2-)Both FDs are turned of at the same time.
3-) The reason is :
From Airbus FCTM :
- Both FDs must be disconnected once APs are disconnected:
‐ To ensure autothrust speed mode
‐ To avoid possible confusion between FD bar orders and, TCAS aural and VSI orders.