As per http://www.postgresql.org/docs/9.4/static/runtime-config-replication.html#GUC-SYNCHRONOUS-STANDBY-NAMES at any time the primary waits for ACK from just one standby server in the list. (The first one, and if its not available, the second one and so on). So even if something goes amiss with the synchronous slave, the secondary slave will become synchronous provided its listed in synchronous_standby_names.
Payal Singh, Database Administrator,
OmniTI Computer Consulting Inc. Phone: 240.646.0770 x 253
Synchronous replication offers the ability to confirm that all changes made by a transaction have been transferred to one synchronous standby server. This extends the standard level of durability offered by a transaction commit. This level of protection is referred to as 2-safe replication in computer science theory.
Does this mean that Synchronous replication only works with one standby?