Secondary professions for a Dervish

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

Example Builds D/W Whirling Dervish

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

Also, Symbiosis can be set down for a large health increase when enchanted.

Dervish Elementalist
Dervish/Elementalist could put Conjure Lightning/Flame/Frost to good use while having equipped a scythe of the same element, 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 armor and gain other significant boosts. A Dervish/Elementalist specializing in Water magic could use Frozen Burst or Water Trident to slow down fleeing foes or he could use PBAoE skills (like Inferno, Flame Burst etc.) to deal substantial ammounts of damage to foes he fights.

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.

Channeling would seem to be a good energy management spell to use, as it is an enchantment and the dervish casts spells close to enemies. Prophecies will be needed for this combination though.

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. 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
Like any melee character, the Dervish could use Plague Touch. Barbs increases the damage dealt to his current target.

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

Dervish Ritualist
The Ritualists' ashes could give the dervish a quick healing boost. And it can use more cool charges from the moon!

Dervish Paragon
A Dervish/Paragon is a good choice when the goal is unlocking all the new skills fast, for use in PvP. Scythes also add adrenaline for all three hits, charging up shouts faster.