A common shortcut in 240 V panel design: install two single-pole 120 V breakers, clip a tie bar across the handles, and call it good. It looks clean. It saves a few dollars. And it can leave one leg of a 240 V circuit fully energized when the other trips, turning a routine service call into an equipment failure or a personnel hazard. Two 120 V breakers do not equal one 240 V breaker, no matter how the math looks on a meter. The reason has to do with how split-phase service actually works, what a "common trip" mechanism does that a tie bar can't, and what ABYC E-11 and UL 1077/489 require for any 240 V circuit on a vessel or specialty vehicle. This article breaks down the engineering, the standards, and the failure mode — with a real-world panel comparison from Paneltronics that shows where this mistake happens and how to avoid it.