Does the Cube encoder stream RTMP to the primary and backup server at the same time?

A single Cube encoder can only stream to a single destination at a time.

When a Cube encoder is configured to use the RTMP streaming method stream and both the Primary and Backup URLs are configured, only one is used and actively streaming at a time.

The encoder will only try to connect to the Backup URL when it fails to resolve (from a DNS lookup) or fails to connect to the Primary URL.