The reason why the Latin prefix for "twice" is "bi-", while the Latin word for two is "duo", is exactly the same sound change (dw -> b) that turned archaic Latin "duenos" into "bonus" in Classical Latin.
Thanks, I wondered that--when I first read it ("read" in the sense of sounding it out and looking for words I might know), 'duenos' reminded me of the Spanish for "owner". I was puzzled to find it translated "the good (man)" instead, but this explains it. I guess dw --> b makes some sense, too. So why did 'duo' not become bo?