What happens if two process try to REFRESH MATERIALIZED VIEW CONCURRENTLY at the same time?
As mentioned in this answer, "REFRESH MATERIALIZED VIEW CONCURRENTLY takes an EXCLUSIVE lock" on the table. Following the crumb trail to documentation we can read that an EXCLUSIVE lock on a table "allows only concurrent ACCESS SHARE locks, i.e., only reads from the table can proceed". In the same paragraph we can see that "EXCLUSIVE conflicts with ... EXCLUSIVE", meaning that another REFRESH MATERIALIZED VIEW CONCURRENTLY statement, which requests the same EXCLUSIVE lock, will have to wait until the earlier EXCLUSIVE lock is released. 

If you want to avoid waiting for this lock for an undefined period, you may want to set the session variable lock_timeout to a sensible value.