Double Hashing with Strings as key How would you choose the second hash function with for double hashing with string as key?
My first hash function is the scalar product of a random int array with the 16 bit number of each char. Is that a good first hash function? And what should I do for the second function now? 
hanks in advance.
There are many hash functions for hashing a string; a web search should turn up a bunch.  You can use any one you like.  The fact that you are using double hashing doesn't impose any additional requirements on the hash function.  Yes, that would be a reasonable hash (and you could use two different int arrays for the two different hash functions); there are many other choices that would be reasonable as well.