I think, you should use replication for this task. It would be more clean and right solution from application architecture perspective.
You might configure 2 databases (with some out of-the-box solution or boiler-plate code). But it would decrease the application performance because each time when app would trigger a query, it must be executed at two DB instances. And in case of transactions, it would get even more complex/slow.
So replication is best way for such task. If you want to use selective replication use Tungsten. Let me know your specific need that can't be met with replication. I might point some more ideas for that.