I have found that the top two class combos are Druid/Rogue and a Warrior/Ranger. Since its easier to do a Druid/Rogue combo, which I use, I will explain that: use your most stable device for the rogue (In my case an iPad 2), and your oldest device for the Druid (iPhone 3GS).
The prime playing position is while laying down on your stomach, and place the DPS device above the Healer.
To move both chars simultaneously, it is good to have the rogue run ahead, have the druid cast bandage wounds, and have the rogue keep running so the Druid follows him trying to get in skill casting range.
While killing bosses, focus mainly on the healer device. The rogue is fine to sit there autoing, but in most cases when he dies your druid is screwed and a big mess happens. After a little bit you will be able to play both chars simultaneously, but in the beginning it's best to keep as much attention on the druid as you can, while casting skills on the rogue if everything on the health front looks good.
I wish you the best of luck, and hope you enjoy the duodevicing life. If you have any questions, asking here or pming me are the best ways to reach me
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