Once you have added channels for all of the replication sources, issue a START REPLICA (or before MySQL 8.0.22, START SLAVE) statement to start replication. When you have enabled multiple channels on a replica, you can choose to either start all channels, or select a specific channel to start. For example, to start the two channels separately, use the mysql client to issue the following statements:

mysql> START SLAVE FOR CHANNEL "source_1";
mysql> START SLAVE FOR CHANNEL "source_2";
Or from MySQL 8.0.22:
mysql> START REPLICA FOR CHANNEL "source_1";
mysql> START REPLICA FOR CHANNEL "source_2";

For the full syntax of the START REPLICA command and other available options, see Section 15.4.2.6, “START REPLICA Statement”.

To verify that both channels have started and are operating correctly, you can issue SHOW REPLICA STATUS statements on the replica, for example:

mysql> SHOW SLAVE STATUS FOR CHANNEL "source_1"\G
mysql> SHOW SLAVE STATUS FOR CHANNEL "source_2"\G
Or from MySQL 8.0.22:
mysql> SHOW REPLICA STATUS FOR CHANNEL "source_1"\G
mysql> SHOW REPLICA STATUS FOR CHANNEL "source_2"\G