Right now, your only option is to run that playbook separately, setting --limit to db1 and db2 respectively.
I have a similiar use case, where i deploy clusters, and want to serially run the deploy on each node of the cluster, whilst running all clusters in parallel.
So that would be:
- step/task 1 on all the first nodes in parallel
- step/task 2 on all the second nodes in parallel
- ...
This is currently not possible in ansible up to 1.9. i am wondering if that could happen with a strategy plugin in v2, but this will probably need an update to the inventory code, allowing the executor to know about the concept of a 'clustergroup'.