Curt, I don't have the book with me but the NEC absolutely does require a common trip breaker for multi wire circuits. This is a double safety feature. If you don't use a common trip breaker and both circuit get put on the same phase, the nuetral may carry up to twice it's intended load(1 circuit pulls 15 amps another pulls 18 amps, then the neutral is carrying 33 amps) - Not Good!
Also both circuits must open at the same time to prevent stray voltage on the neutral when working on either circuit.
If someone doesn't beat me to it I'll cite the code in the morning when I get to the office
