Default binary log position for change_master_to in MySQL circular replication?
(-sorry if I have misunderstood)

(-assuming you are using binlog file and pos).

The key things to make sure of are that your servers have unique server-ids (if you have replication running already then this should be ok).

You also probably want to set:

auto_increment_increment
auto_increment_offset


values so that the two Databases don't generate overlapping primary keys.

http://dev.mysql.com/doc/refman/5.7/en/replication-options-master.html

After that on your Slave DB (B), simply run SHOW MASTER STATUS.

Use those details to issue the CHANGE MASTER statement on DB A.

What you should find is that anything executed on A is then replicated to B and vice versa.

However anything executed on A shouldn't be replicated back from B to A.

And as such, the only queries written back to A will be those that are executed directly on B.

Finally, you will want to set B to read_only to prevent anyone accidentally writing to it, until such time as you want them to.