The danger in your scheme is that an attacker who finds a way to see lots of keyed hash values in one hash table can flood a different hash table. You should keep a 16 byte key per hash table.
Thanks, I'll only have one set of hashes. My question was really is it ok to use one key per table of hashes, or does it have to be one key per hash. I think you're saying one per table is fine, but wanted to be sure I wasn't doing it wrong. I'll have another look for examples.