It is highly recommended to use a Backup encoder in case there are issues with the Primary broadcast.
If you use a single encoder to broadcast to both primary and backup entry points, your set up lacks redundancy. If the encoder fails, both primary and backup broadcasts will cease. To avoid this, consider using two encoders, with one broadcasting to the primary entry point and the other to the backup. However, do not use both encoders to broadcast to both primary and backup entry points, as the two broadcasts will conflict.