Faramir knows that he was not his wife's first love. But she chose him in the end (though perhaps the choice was not entirely hers) and that is all that matters now.
Faramir does not regard Elessar as a threat. The King is happily married, and Faramir knows he never returned Eowyn's love.
And if, sometimes, in her sleep, Eowyn murmurs a name that is not his? Faramir lets it pass, and pretends that he has not heard. Because after all, he, and not Aragorn, is there to hear it.
Eowyn cannot control her dreams, and waking, she is his.