Should french fries be fried two times? Most professional cookbooks tell me to fry french fries two times. First at a lower temperature of about 150-170°C and then at a higher temperature about 180-190°C. 

Reference: http://en.wikibooks.org/wiki/Cookbook:French_Fries#Variations

Related: What varieties of potato would be good for chips / french fries?

What are the advantages of frying the potatoes two times?
The lower (first) temperature actually cooks the potato so that it is tender, the hotter temperature (second) is what gives the crispy golden coating.

If you just did the lower temperature, your fries might be too soft. If you just did the hotter temperature they would be too too tough.