I suspect that implementing this as a pure DBI subclass would prove a bit cumbersome; you'd effectively have to replicate the entire DBI i/f in order to route the API calls to either of your connections.
That being said, you might look into DBD::Multiplex. I've not used it, but it has been updated quite recently and I know
Mssr. Bunce had given it some attention at one time. It would appear to provide most of the functionality you seek.