Storm's End + double dominance Melisandre.......how does that work?

No trick. Dominance resolves, with Storm's End replacing the way Dominance normally resolves (1 for your House) with the way described in its text (1 each for two different characters). Then, you passively claim 1 power for your House thanks to Mel.

(Compare her text to CS-Robert Baratheon if you don't believe her effect is passive. Note the lack of the word "instead" if you think it should be a replacement effect.)