Add database to existing MySQL master-master replication which uses binlog_do_db?
Master-Master is simply Master-Slave in both directions.  So all the settings work the same.

Data does not endlessly loop between the two Masters as long as they don't have the same server_id.

I'm pretty sure you simply have multiple lines.  See https://forums.mysql.com/read.php?26,255998,255998

M-M is fraught with perils, but I guess you have gotten past them.  So I won't harp on it.