(Intel uses "host" and "agent" now instead of "master" and "slave".)
It doesn't sound like you need to do anything other than make the connections. Nios will initially have access to B when needed and then X will have access since the Nios is not accessing it. Arbitration is only an issue if multiple hosts are accessing a single agent at the same time.
If you are saying that Nios and X may access B at the same time, then you can adjust the arbitration shares for one or the other to give higher priority or switch to the fixed arbitration scheme mentioned.