Split WP install between 2 databases?

Is it possible for WordPress to work from two databases?

The reason I ask is that we’re approaching our 100mb limit for database size on our host (1and1) but have up to 100 databases, so what I was hoping to do is essentially ‘add on’ another database for when the limit is reached?

Yes, but that is out of quick and easy realm.

See HyperDB in Codex and repository for starters.