The split California king mattress is exactly as it sounds: It's essentially two separate mattresses that equal the same dimensions as a single California king.
The split California king can be a good option if you share your bed with a partner and have different firmness or temperature preferences, or if one of you is a restless sleeper.
If you upgrade to an adjustable base, you can further personalize your sleep experience without disturbing your partner. However, if you tend to roll toward the middle of the bed, you may find the split California king uncomfortable.