It's standard to have at least two hashing functions. You can then simulate having x hash functions simply by combining those two.[1]

But, your question is entirely valid and I'm not really sure why you couldn't do that. It might simply be that the kinds of hash functions used in bloom filters (non-cryptographic) are not well distributed.

[1]: http://citeseer.ist.psu.edu/viewdoc/download;jsessionid=4060...