I dont think this is possible.
But the node with the lowest advertising frequency will be master.
So you could per default have the master on lets say 4 and the slave on 5.
<– 4 is master, 5 is slave
If the master goes down the slave takes over.
<-- 5 is master
Now before you connect up the new box you set it to 6.
So you have to set it manually to 4 to take over.
<-- 5 is master, 6 is backup
manually change to
<-- 4 is master, 5 is backup