One of the best known is Top Language Jobs. They are used by some high level employers, and the London-based Multilingualvacancies is also worth a closer look.
However, you will probably save a lot of time if you do a more exact internet search. Let's say your two languages are English and French; if you put a phrase like 'french english bilingual jobs' in a search engine you will get links to some more specialist websites such as this one. Or try the phrase 'french speakers uk opportunites' or similar. It can also help to do a search for specific job types, such as translator or interpreter.
I would also suggest contacting a regular employment agency or going to your local careers advice centre and asking about the kind of job opportunities that being bilingual in a specific language can give you.