Although it may seem like a pain, you might try separating them for about three weeks and then gradually reintroducing them. If you haven't tried a Comfort Zone plug-in with Feliway, you might want to get that, although they are sort of pricey.

Actually as long as they are not really fighting, the hissing may be a normal reaction and may stop over time. Sometimes cats just need to work things out on their own.
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Smile, it will make them wonder what you are up to!

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