http://www.vibramfivefingers.com/technology/faqs.htm
I WEAR ORTHOTICS. CAN I STILL WEAR VIBRAM FIVEFINGERS?
FiveFingers footwear is designed to promote better foot mechanics— naturally. An orthotic is an assistive device to support your foot’s function and help you compensate for any biological aberrations. If you have an extremely flat foot, you will need to work into wearing your FiveFingers® gradually, as your muscles will need time to adapt and strengthen.
CAN I USE MY ORTHOTICS IN MY FIVEFINGERS?
FiveFingers are designed to be worn directly against the sole of your foot. Putting something between the two would defeat their purpose.
i do 100m sprints in my vibram kso's all time. it's gonna take some time for you or anyone to get used to running barefoot/vibrams. you will be using muscles that you have never really used before. and you have to toughen up the soles of your feet. you don't need 2 pairs. just buy the kso's and you'll be fine.