One Primary and two standbys

I am about to build/add a second standby to join the primary, and then test a switching role as part of the project objective.  I have the following scenario that trouble me because I wasn't able to explain to myself based on information I have so far and hope someone could share the experience or explain.

In my current  primary and one standby system which has already been setup and working, so far I am able to configure the tnsname and listener files without using broker to make it happen(first phase of testing).  If I add/create second standby, I just don't how the following situation will work, but I know it has got to work, it is just I don't know the main mechanism or setup behind which I try to understand and find out.