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 Sycthe.

Dervish/Ranger
Apply Poison is an extremely useful skill for a D/R, as it can be used to spread Poison among adjacent foes very rapidly.

Dervish/Elementalist
There can be some useful skills in the earth magic line as it contains defensive enchantments such as Armor of Earth, Stoneflesh Aura and Obsidian Flesh (not to mention Sliver Armor) for Tanking in the Domain of Anguish. Also, PBAoE skills such as Aftershock, Inferno, and Shock 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. Also see the 135 hp Dervish build.

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

Dervish/Necromancer
Like any melee character, the Dervish could use 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 used to counter blind long enough to spike an enemy. Weapon spells are also a boon to this profession combo.

Dervish/Paragon
This profession combination provides little benefit for a Dervish primary.