Add database to existing MySQL master-master replication which uses binlog_do_db? We have 2 Mysql 5.7 servers running in master-master replication mode.

And there is a database replicating between these 2 servers:

binlog_do_db = database1


Now we need to add a new database into replication, I have read Add database to existing replication which uses binlog_do_db?, but its for master-slave. Is there any other things should be considered for master-master situation?
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.