I got my anemone crab this way.
Are the clowns the same size? How big is the tank they're going in? Are you sure it's a sebae clown and not a clarkii?
If the clowns are different sizes, I'd put one of them in an acclimation chamber, or hamster ball. Then add it to the tank with the other clown, to see how they react to each other. After a day or two, if they haven't tried to kill each other, I'd release the confined clown.
The fact that they are different species does not prove they won't get along. Different species have been found in the same anemone, and they do hybridize. I would proceed with caution though.