My favorite Twins deck builds use Arwen as the third hero. (Well, my favorite use of the Twins is actually to separate them, as in this fellowship: http://ringsdb.com/fellowship/view/19/separated-at-birth... But anyway.) Leadenethor has some potential as the hanger-on, though: he resource-matches with the twin you generally care more about keeping rich, and there's the usual rapid start and relatively low starting threat that is always his advantage.
I had never used Heir of Mardil in a deck before, but boy oh boy can it be powerful. With four different ways of adding resources to a hero -- Steward of Gondor, Captain's Wisdom, Errand-rider, and Denethor's ability -- there are plenty of opportunities for the once-per-round readying effect. Rod of the Steward is pretty inefficient, but sometimes you've built up a ton of resources on Elrohir and heck, there aren't that many enemies on the board right now, but you really need that Feint or Sneak Attack sooner rather than later...
(Black Arrow may seem a little too combo-y here, since you need a Rivendell Bow in play before you can attach it, and that's only a 2x; plus, Bow of the Galadhrim can't be attached at all. Explanation: I usually played this deck paired with a Haldir one, so there was always a good target for these cards; consider dropping them for something else if it feels weird, or going up to 3x with the Rivendell Bow.)
The problem with Bow of the Galadhrim is that it doesn't attach to Noldor.