Don't think of it as a split. That's main point StarCraft II lead producer Chris Sigaty was trying to get across during our brief interview following the StarCraft II panel at BlizzCon where it was announced that the single player campaign would be delivered as three separate products - first Terran, then Zerg, and finally Protoss. "Effectively each game in the will be an expansion," Sigaty explained when I told him of our reader's initial concerns about buying the same game three times with a different single player component. Think Warcraft III's The Frozen Throne, or the original StarCraft's Brood War.
Each new single player campaign will come with upgrades and changes to the multiplayer. The main difference here is that instead of giving you a tacked-on extension of the single-player experience as you often find in an RTS expansion, each title comes with a fully realized campaign.