Secondary professions for a Dervish

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

Wild Blow is also very often used by Dervish/Warriors, while not using any other adrenaline skills. This skill is especially good at ending stances like Escape, Dodge, Defensive Stance or Dryder's Defenses.

Dervish Ranger
Apply Poison, like with other melee classes, would be a good choice, especially since the scythe hits adjacent targets, spreading the poison around each target.

Dervish Elementalist
Dervish/Elementalist could put Conjure Lightning/Flame/Frost to good use while having equipped a scythe of the same element (such as the Ice-element preorder scythe), resulting in decent extra damage to up to 3 adjacent foes.

A Dervish/Elementalist could utilize Earth Magic such as Sliver Armor or Ward Against Melee to bolster his/her armor and gain other significant boosts.

A D/E specializing in Water magic could use Frozen Burst or Water Trident to slow down fleeing foes or she/he could use PBAoE skills (like Inferno, Flame Burst etc.) to deal substantial amounts of damage.

A Dervish can also make good use of Glyphs, as they will augment his or her enchantment spells. In particular, Glyph of Lesser Energy can greatly ameliorate the Energy costs of using multiple enchants.

Example Builds:
 * Build:D/E Mirror of Ice Scythe

Dervish Mesmer
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.

Spending a lot of his time casting enchanments in the middle of a melee fight, a Dervish could make could use of Channeling. With this cheap and long lasting enchantment and Mysticism, the character would gain energy both when the enchant starts and stops. However, Channeling is a Prophecies skill, so it's not available from instructors in Istan (at least).

Example builds:
 * Build:D/Me Dervish Solo FoW Farmer

Dervish Monk
Probably this could resolve in a nice combination of Mysticism 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. Healing Prayers spells are less useful, however. Dervish already have several self-healing skills that can equal or surpass the healing skills typically used by a melee/Mo combination.

Many smiting skills involve point blank Area effect damage, so could be a useful addition to dervish damage. Many smiting skills are also enchantments, making them useful for powering other dervish skills.

Example builds:
 * Build:D/Mo 130hp Dervish

Dervish Necromancer
Like any melee character, the Dervish could use Plague Touch.

Aura of the Lich could also prove to be of great use for a Dervish (instant healing in combination with enchantment removal skills, in addition to its usual effect). For "blood-drinkers", Blood Renewal can also do the same job, without taking the elite slot.

As a condition-dishing machine, Dervish player can find Tainted Flesh or Poisoned Heart (if quickly removed) quite an interesting alternative as well.

Dervish Assassin
The shadowstep abilities, knockdowns and conditions like the Deadly Arts make valuable additions to any melee-focused Dervish, especially in PvP.

Shadow of Haste is an alternative for those who do not want to invest in Wind Prayers.

Example builds:
 * Build:D/A InvinciBomb

Dervish Ritualist
A Scythe wielding Dervish using Weapon Spells have increased effectiveness. Guided Weapon may also be used for attacking multiple enemies to ignore blocking skills. Weapon spells are also useful since Dervish skills often require the sacrifice of an enchantment. Since weapon spells are not enchantments they will not be removed accidentally.

Dervish Paragon
Scythe AoE effect will net 3 strikes of adrenaline, charging up shouts fast. Also, Paragon has a reusable resurrection signet and no-attribute based condition removal signet with fast recharge, which can be pretty useful.

Another useful method of this combination involves being equipped with a spear and shield, using Paragon skills to attack and Dervish skills to heal yourself.

With a small to moderate investment in Command, "Go for the Eyes!" can notably increase your damage thanks to the high max damage of scythes.

You can also unlock both professions skills in PVE without playing it twice.