I suggest the PBSFs blinking on opposite cycles of each other. You'll get a superbright LED flash twice as often. To get a good rhythm between the two lights, you first need to determine the length of one flash cycle. Turn a light on flash, and watch the pattern until you can count with the main flash of the light (like in music, 1, 2, 3, 4...1, 2, 3, 4, etc). Now that you have the tempo down. Start one light and then start the other light halfway between beat 1 and 2 (an eighth note if you will), and you'll pretty much have the LEDs flashing opposite of each other.
It's harder to accomplish this is you turn one light on, watch it for a while, and then turn on the other light. This is because there is a delay in the circuitry, so the light doesn't start flashing right away when you press the button.