WPMUdev has a Multi-DB plugin but you won’t see any performance gains from splitting your database up unless you have a very high volume of sites with thousands of tables.
From the plugin instructions:
Step 1: Decide how many databases you
want (16, 256, 4096)So, how do you know that this plugin
is necessary for your install? Well,
there are several factors to consider
beyond the scope of this walkthrough,
but as a basic guideline:1-5,000 blogs/sites you should be fine
with your WordPress default database5,000 – 50,000 blogs/sites – go with
16 databases50,000 – 100,000 blogs/sites – use 256
databases100,000+ blogs/sites – use 4096
databases