For the bicycle, one on the handlebars and one on the helmet. I'm thinking there could be some good cutaways to ad interest to videos that way. We'll see.
for downhill MTB, handlebar and helmet cameras are all you need. if you intend to use your video as evidence (road rage, unsafe passing, tailgating, etc), add a rear camera.
for road racing... it could go either way. if you're always in the back, a rear camera isn't much use
