Once you have added channels for all of the replication sources, issue a START REPLICA | 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 | SLAVE command and other available options, see Section 13.4.2.7, “START REPLICA | SLAVE Statement”.

To verify that both channels have started and are operating correctly, you can issue SHOW REPLICA | SLAVE 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