two aren't absolutely necessary, but i won't go back to one, now that i've experienced the sheer bliss that is two.

you just take the second toe strap and thread it through the front holes of the pedal and loop it up around the top of the clip, it's not really looped through anything on the clip. i usually use some old straps that already know what shape they should be in. so you have the main strap that wraps around somewhere near the middle of your foot, and the front one that wraps around farther forward. it feels really, REALLY secure, but i've always been able to pull my feet out fairly easily. i started doing it after i snapped a clip where they attached to the pedal. the second strap really helps out

the pic is kinda dark, but you can sort of see what i mean...