with food... as on an empty stomach it may act as a laxative.

It is absolutely not necessary to take the B12 in divided doses. You could take it once in the morning if you wanted.

Calcium, zinc and mag are controlled by cellular active transport mechanisms, so dividing up the doses is better. Dividing up water soluble vits like C and B complex also is better since they are excreted quickly.

But B12 in large doses is passively absorbed in the small intestine, like a drug.
Less of a critical thing for timing or dividing up the dose.