PostgreSQL streaming replication with slots works for one slave, but not for another slave
Community Wiki answer generated from a comment left by the question author. Feel free to edit to improve

Nevermind, I fixed it. There needs to be one replication slot per slave. I added another replication slot "master2". So slave #1 is using the "master" replication slot and slave #2 is using the "master2" replication slot. Works fine now.