I have a 2 node replica set. 1 node is supposed to always be primary and the other is for backup purposes.

My rs.conf() looks like that:
{
"_id" : "shard0001",
"version" : 6,
"members" : [

{ "_id" : 0, "host" : "one:27017", "priority" : 2 }

,

{ "_id" : 3, "host" : "backup:20002", "priority" : 0, "slaveDelay" : 10800, "hidden" : true }

]
}

The problem is that when backup goes down, "one" starts thinking that it's secondary, because it's afraid that backup will start believing that . Such fencing mechanism should not be valid when other node is hidden and priority 0. Or your docs are wrong by saying that priority 0 node cannot ever become a primary (http://docs.mongodb.org/manual/tutorial/configure-secondary-only-replica-set-member/).