MySQL: Replicate data within 2 same schemas on 1 mysql server
You could setup a script to read the binary log, convert it to SQL (with the utility mysqlbinlog) and change the schema on-the-fly before applying the changes. You could also setup triggers on every table to INSERT/UPDATE/DELETE every row on the copy schema. But that is a lot of work to do it right.

Running 2 instances of MySQL on the same machine is very easy, and may be closer the the production deployment. Tools like MySQL sandbox makes running "n" different instances of MySQL even easier, taking just a command to setup an instance or a complex replication topology! You can even simulate replication lag with MASTER_DELAY or pt-slave-delay to see how your application behaves when the state of both schemas is not the same.