Dual Implement Spellcaster lets you add the bonus from an off-hand implement to an attack using your main hand implement. I am making a Human Marauder Ranger and I would love to be able to add an off-hand weapon bonus to a main hand attack. I don't expect such a thing to exist (it would be too good), but does anyone know for sure? Thanks.
Dual-Implement Spellcaster only adds a damage bonus, not an attack bonus; I presume an analogous feat would do the same, and that it would disallow off-hand attacks as well. Given that point, there are quite a lot of feats that add extra melee damage alone, although you won't get the flavor.
However, you may well be better off with existing powers anyway. Assumptions:
The melee basic attack with the feat would get you D + S + E + E (the last is the bonus from the feat). Two hits with Twin Strike get you D + E for the first and D + E for the second. So if D is bigger than S, then you're better off with Twin Strike. Given that S tops out at about 10, and D starts out at 7.5 in epic just from using longswords and Weapon Focus, it wouldn't take much optimization to make Twin-Striking better than the feat!
I have not seen such an ability on any official WotC D&D book.
I believe the reasoning behind the feat for spell casters as opposed to weapon wielders is because you are always channeling the spell through your wands, orbs, etc. rather then the items are casting and dealing damage with the spell themselves as opposed to where a weapon wielder is dealing damage with the weapon itself. Additionally, as far as I know, there are no spells that require an off-hand implement to cast the second part of a spell (if there is any) as opposed to dual wielding options for weapon wielders where if you wish to dual wield you have the option of abilities to use both weapons to make one attack with the main hand then another with the off hand. Spell casters just do not have this option. I guess the idea was if you want to look cool dual wielding wands, spell daggers, etc. they would give it to you in the way of enhancing your normal single cast spell.
Of course if I have missed this option somewhere I will delete this answer, I just haven't seen or heard of it anywhere.