@evan You asked "should", not "does", so my answer is "only people who follow *both* Alice and Bob", that is, the intersection of both sets of followers. (Your 3rd option, to me, reads like "people who follow *either*", that is, the union.)
Whether Alice-only follower Carol (resp. Bob-only follower Dave) should gain access to Bob's reply by following Bob (resp. Alice) after Bob's reply is unclear to me.