Secondary professions for a Dervish

Dervish Warrior
The Dervish/Warrior Class is a very potent combination. Combining the Dervish Elite Avatars with the Warrior’s melee attacks and stances can make an effective build.

Dervish Ranger
A Dervish/Ranger could use Apply Poison to dish out poison in an area of effect.

Dervish Elementalist
A Dervish Elementalist can benefit from the numerous enchantments (Armor of Earth, etc.) and AoE spells (Aftershock,etc.) offered by his second class to endure and deal more damage in combat and gaining benefit of his Mysticism attribute.

Dervish Mesmer
A Dervish/Mesmer could use Illusionary Weaponry and cause armor ignoring AoE damage.

For farming or mass AoE damage, another good combination is Signet of Midnight (which also blinds the Dervish) coupled with Epidemic for spreading blindness. Using Sand Shards while blind makes way for repeated, heavy damage. Since Signet of Midnight works for the same time it recharges (15 seconds), one could technically farm forever, with only Faithful Intervention or Watchful Intervention for healing. Whirling Charge can speed up this process as well.

Dervish Monk
Probably this could resolve in a nice combination of Mysticim combined with Protection spells to get massive amount of energy back from your cheap protective enchantments. Contemplation of Purity could quicken this, and since many Dervish enchantments give a Dervish even more health upon ending, incredible spike heals and conditon removal is possible. You can also use skills such as Vigorous Spirit in conjuction with any Scythe Mastery skill- Dervish Scythe Mastery skills hit up to 3 adjacent opponents, so as you would get massive amounts of health from Vigorous Spirit. This could be very effective in farming.

Dervish Necromancer
Spinal Shivers combined with wind attacks/enchants for cold damage is a nice combination.