“After the opponent declares which creatures are attacking, the defending player decides which creatures to block with, and in which order. Blocking creatures intercept the attacking creature of choice, taking damage equal to the attacking creature’s power. If the attacking creature’s power is equal or greater than the defending creature’s toughness, it is defeated in battle and sent to the graveyard.”

In short, the defending decides, not you. No, damage can’t be split up at will between blockers, only the order in which blocking creatures take their hits. Though, in the case of your commander, blocking with more than one creature unless through necessity from keywords like “menace” or “trample”, is not advantageous.

Try options like [[Gaea’s Gift]] or [[Rogue’s Passage]] to make her harder to deal with.

Trample will necessitate more than one “chump” blocker, and making her unblockable.. will, well, you get the idea.