A Double Wed witch has forsaken other powers to expand his primary witchery. The effect varies, depending on the manifestation. A witch can become double wed at any point he would normally gain an additional manifestation type.

The effect of being Doubly Wed varies, depending on the manifestation.
A manifestation that has a number of uses per day gains 50% more uses (so a 4/day power can be used 6/day).

A manifestation with one use per day goes up to two.

For Blade manifestations, the witch gains the ability to manifest a second blade identical to the first, or to manifest a more powerful single blade (Chosen when the manifestation is picked). An iron witch gains an additional bonus to hit (but not damage) of +1/4 levels, a wind witch gets a deflection bonus of +1/3 levels, rather than 4, a cold witch deals an additional 1.5 points of cold damage per 2 levels rather than +1 (round down), a mind blade deals 1d6+1.5/level, a wood witch deals 1.5*wisdom mod in bonus damage, and counts 1s and 2s as 3s when attacking undead, and a Sea Witch sucks just a little less.

Some manifestations don't fit either mold, so their effects are below:

Armor Song: The Iron Witch can manifest a shield as well, which has a shield bonus of 1/2 wisdom mod, and an enhancement of +1/6 levels), and also has no armor check penalty. The shield may the witches' size or smaller.

Nature's Warding Word: In addition to a natural AC, the witch gains DR 1/- for every 6 levels (yes, taking it as your 2nd manifestation gets you squat. This is intentional)

Fire Lance: A fire lance deals an additional 1.5 points of fire damage per 2 witch levels.

Sea Song: The chance for hostile encounters is 25%, rather than 50%.

These abilities stack with Witchery Lord, but the Witchery Lord is applied to the base ability and then the Double Wed bonuses are added on.