Secondary professions for a Dervish

Dervish/Warrior
Armor-boosting skills in the Tactics skills make good additions for fragile Dervishes. "Victory is Mine!" works well with condition-causing Dervish skills.

Wild Blow is also very useful, as its drawbacks are easy to avoid and critical hits are more powerful with a Scythe.

Dervish Ranger
This profession combination provides little benefit for a Dervish primary.

Dervish/Elementalist
The defensive enchantments of the Earth Magic line, such as Armor of Earth, Stoneflesh Aura and Obsidian Flesh, allow exceptional Tanking in the Domain of Anguish.

PBAoE skills such as Aftershock, combine well with the Dervish's high DPS to adjacent foes.

Dervish/Mesmer
Signet of Midnight coupled with Sand Shards causes repeated, heavy damage. Epidemic can be used to spread conditions caused by scythe skills.

Channeling, a cheap and long lasting enchantment, and Mysticism provide ample energy for combat casting builds.

Dervish/Monk
Mysticism with Protection spells grant massive amounts of energy back from cheap protective enchantments; however, Dervishes already have several self-healing skills that surpass the healing prayers line.

When not using Avatar of Melandru a dervish can deal with conditions using Mending Touch.

Dervish/Necromancer
Like any melee character, Dervishes can benefit from Plague Touch.

Tainted Flesh or Poisoned Heart, when combined with any enchantment ending skill, can spread conditions easily.

Dervish/Assassin
Shadow stepping Skills such as Shadow Walk are useful to teleport up to casters in the back lines. Shadowy Burden's snaring and armor reduction effects combine well with Vow of Strength. Dash can be used as a no-attribute speed boost.

Dervish/Ritualist
In PvP, Sight Beyond Sight is often paired with Extend Enchantments for immunity to blind.

Dervish/Paragon
This is also a good way of UAS for the Paragon and Dervish in one run of PVE. A very good thing if you just need the skills for your heroes. Otherwise this profession combination yeilds little benefits for the dervish primary.