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.
Snippets of code should be wrapped in
<code> tags not<pre> tags. In fact, <pre>
tags should generally be avoided. If they must
be used, extreme care should be
taken to ensure that their contents do not
have long lines (<70 chars), in order to prevent
horizontal scrolling (and possible janitor