Celtic Heroes

The two unique spells to consider with a dual mind set is Bolas and Root.
Root does not work well with melee class, so with that in mind Druid with root goes best with a DPS Mage.
Bolas can work with melee, but it is best also with ranged DPS, so again I would pick Ranger with a Mage.
If either root or bolas fails, energy shield can usually hold out till the next recast.

The fastest leveling combo would be a Rogue and Druid. Max Bark and keep all you other points in vines, lightning and touch and let the Rogue go crazy with combos and you will be surprised how fast you can level.

Mage/Ranger

Just for fun I did Druid/ Druid I found very interesting. (One was support only). Both only early 100's but was cool to try.

The two unique spells to consider with a dual mind set is Bolas and Root.
Root does not work well with melee class, so with that in mind Druid with root goes best with a DPS Mage.
Bolas can work with melee, but it is best also with ranged DPS, so again I would pick Ranger with a Mage.
If either root or bolas fails, energy shield can usually hold out till the next recast.

The fastest leveling combo would be a Rogue and Druid. Max Bark and keep all you other points in vines, lightning and touch and let the Rogue go crazy with combos and you will be surprised how fast you can level.

The fastest leveling combo is rogue rogue if you can afford the lix, pots and idols.

Have quad boxed 4 rogues to 40 so far it's funny hitting shadowstrike after shadowstrike on a mob.

Well for bossing its tough to say.. but for leveling I like ranger and rogue. Rogue can dps alot and use distract while ranger bolas and heals more. Rangers can also buff rogues damage.

Rogue/Mage duo is awesome too (much prefer it to Rogue/Rogue).

I've duo'd Mage/Druid back in U3 with my Full Radiant melee-mage.. Was awesome, but still not quite on level with Rogue/Druid.

A fire mage and a Rogue do very well together, but not duel devicing, get a friend. Why fire mage? They deal damage quicker so more dps.

You might wanna go with a threesome, a hybrid warrior(bordering on tannish tho) a full on dps rogue, and a druid woth group heal hope i helped

I would like to see someone try druid/ mage
The druid can keep his and the mage's health up (and assuming I have the correct idea about these skills) the mage can keep sigil requirements low, the plus side to this is the druid can (often times) join endgame clans at lower levels and they can both be geared pretty quickly.

I started lvling my mage with clan's low lvl druids only, i thought i'll have to buy like tons of energy sigils and struggle even at low lvls, but instead i just took druids, maxed my fire skills (half maxed lure), put all the extra pts in sacrifice (+2 from frost gloves) and my energy is always full, the druid heals me, the dps is great, i dont need to use anything besides xp (sometimes wisdom when my fire magic needs to catch up).

If you want to level cheaply, use your higher level warrior to tank mobs who would do a lot of damage on your rogue/mage/... alts (trolls for example) The warrior needs to be outside group with enough points in taunt. Another nice thing about this is that your tank will do some damage too.
Thanks to this method i level a lot faster, and never used a single resto on my 173 rogue while only using sk and haste lix.

I tried several pairings for training from 0 to 40 to get a feel of what works best, but Rogue/Ranger leveled the easiest for me. Ranger would bolas, rogue can auto mob few a few seconds, then skills kill on both, Ranger uses light heal on which ever toon took the most damage, although I was heavy on sigils for both once they were over 140.

Was by far the fastest and cheapest way to dual box a pair of toons in my opinion. Ultimately though any pair with semi decent lux and proper build should be fairly effective.

I got my second device when my ranger was already lvl180 so I didnt level my rogue and ranger together. I dont know if they're efficient in lvling together but are certainly efficient in almost evry other aspect. Rangers have complained alot that we are not best at anything, but that comes in handy if you're dual boxing with a rogue. Bossing and lvling both become cheaper and a little easier cause of high defence on rogue and the light heal on ranger. Try and see