This came up in a game yesterday.
One B-wing with FCS fired his primary weapon on a Tie-Fighter.
Usually that means that in can immediately acquire a target lock on the Tie, but there was also Captain Kagi in range.
In this case, since Kagi's ability is a "must" and the FCS is a "may" what should happen?
a) The B-wing must lock on Kagi
b) The B-wing may lock on Kagi
c) The B-wing can not lock on since it may only lock on the defender and "must" triumphs over "may".
d) The B-wing may lock on to the Tie and ignore Kagi.
I went with c)