Two of them would look like you had the brakes on since
they are just as bright as brake lights (in fact, I
thought it used a brake light bulb). I would think having
both on would be dangerous since people might then ignore
your true brake lights (few U.S. drivers have ever seen a
rear fog light).

When I flip it on almost every time the car behind me
backs off -- bright light means brake lights for most
other drivers. In fact, I find this light very useful
for tailgaters.