Should a bus tie breaker be used in series?

Using two breakers in series provides additional redundancy in terms of being able to still break bus parallel if one bus tie breaker should fail to open on command during switching. Breaker fail schemes also benefit from having a second breaker.

Do I need a cable tie or a bus duct?

Busduct or cable tie - depends on the layout. If lay out permits, bus duct is preferred for obvious reasons. Breaker at both ends or only one end - if the intent is to keep the two busses always tied / treated as a single bus, there is no need to have circuit breakers. Just one disconnector will do. Busduct or cable tie - depends on the layout.

What happens if a bus tie is open?

In such a case, when the tie is open, the respective bus incomer protection acts as back up to the outgoings of that busbar and when the two buses are tied, the incomer protection will have to act as back up to all the feeders included in Bus A & B. The tie cable or busduct will remain live from one bus with breaker open at the other end.