Hi Susan,

I take it the knives were originally held together with glue rather than a mechanical bond like rivets, right?

Given that assumption, you're looking for a glue that will hold up to heat and water. I'd say you probably want a two part epoxy for that job; as far as I know that's the best glue you can get for your particular application. That's the kind where you have to mix together two different liquids that make up the glue.