1 P7 will be plenty enough for most riding, though always useful to have an extra light for the fast or tricky descents!
Personally, I’d mount both on the bars, and get a smaller Cree R2 torch for head mount use. The Cree R2 LED has more of a focussed beam, and really penetrates whilst the P7’s are great floods. The R2 will also last 3 times as long on the same battery.
With 2 P7’s on the bars, you can then use full power on one most of the time, then when the battery dies out, you can just switch the other one on instead. Both at the same time would really only be desired for any full on nighttime DH action really.
Will let you know my experience though, as since my last nightride with 1 P7 light and an R2 on my head, I have received another P7 torch from dealextreme! 😉