This has been discussed many times and many (most?) people agree that two lights both front and rear not only makes you more visible but protects against unexpected failure or loss of one light.
I always run one of each pair static (to help drivers judge your distance) and one flashing (to attract attention amongst all the lights on our roads). Strategically-placed reflectives may enhance visibility in addition to lights (I recently posted a link to research that suggests reflectives on the knee and ankle are best).
Some time ago I found an article by a lawyer that implied that if you were involved in an RTC and the other party's lawyer tried to pin the blame on you because you weren't fully compliant with the lighting regs, the court would be likely to look at whether the spirit of the law rather than the letter of the law was being complied with. For example, if you had an adequate relective slap band on your ankles (as mentioned above), it wouldn't matter that your SPD pedals didn't have reflectors.