Secondary professions for a Dervish

Dervish/Warrior

 * Rush can be used as a speed buff. Dervish speed buffs are generally weak.
 * Wild Blow is also very useful, as its drawbacks are easy to avoid, and critical hits are more powerful with a Scythe.
 * Distracting Blow is a useful interrupt.
 * Flail and other IAS skills can help overcome the slow attack speed of a scythe.

Dervish Ranger

 * The skill Serpent's Quickness is useful to lower the recharge time of scythe attacks.

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 more difficult areas of the Nightfall campaign, such as the Domain of Anguish.

Dervish/Mesmer

 * Mesmer interrupts are useful for disrupting skills such as Aegis.

Dervish/Monk

 * Dervishes can benefit from Mending Touch.

Dervish/Necromancer

 * Necromancer Orders et al work well with Mysticism.

Dervish/Assassin

 * Shadow stepping Skills such as Shadow Walk are useful for coordinated spikes.
 * Dash can be used as a no-attribute speed boost.
 * Disrupting Dagger is a useful interrupt.

Dervish/Ritualist

 * Sight Beyond Sight can be paired with Extend Enchantments for immunity to blind.

Dervish/Paragon

 * No benefit.