One blinking, one steady. If both are blinking, and they are both "off" at the same moment, you are invisible , even if it's just for 1/5th of a second, you are momentarily invisible.

Also, I have seen LED tail lights on other bicycles, while I was driving my car, and I have good depth perception, but with the blinking light and no steady light (to focus on) , it is hard to tell where the bike is going. I glance ahead to see if there is oncoming traffic, and I glance back to look at the bike, but the "Blink" is OFF, and I can't see the bike at all.

Again, one blinking , one steady, and a third light on the helmet (blinking by day, steady at night).