The nozzles on The Duplicator's Dual-Extruder are aligned at our factory, and the difference in height between them should be no more than 0.3 mm -- about the height of one printed layer.
We don't recommend adjusting the nozzle yourself at home, but if you do want to fine-tune your nozzle heights, here's how to insert a shim between your Dual-Extruder's bar mount and the extruder carriage.
Here you can see that the right nozzle isn't quite touching the platform:
We're going to slightly raise the position of the Dual-Extruder assembly with the lower nozzle so that the nozzles are more even.
Unscrew the bolts attaching your Dual-Extruder assembly to the extruder carriage and remove the extruder carriage. Turn the Dual-Extruder assembly over. Note the long metal block in the middle of the assembly -- that's the aluminum bar mount. You're going to layer Kapton tape over the side of the bar mount with the nozzle that you want to raise. Doing this will create a shim that will raise this side of the Dual-Extruder assembly's position on the carriage (effectively leveling out the height of your nozzles).
Cut a small piece of Kapton tape and lay it over the end of the bar mount.
Then lay another small piece on top of that. Start with three or four layers of tape -- if that's not enough, you can always add more later.
Use a knife to cut a hole in the tape over the hole in the bar mount -- that's where the bolt that attaches the Dual-Extruder assembly to the extruder carriage goes in, and you don't want to block it.
Make sure the tape over the hole is cut away completely; hanging pieces of tape can get in the way of the bolt when you try to re-attach the Dual-Extruder assembly.
Now you can bolt the Dual-Extruder assembly back on. Raise the platform all the way and check to see if the nozzles are more level.
If you've added the right amount of tape, they'll be nice and even:
If you haven't added the right amount of tape, don't worry -- just unbolt the Dual-Extruder assembly again and add or subtract a couple of layers of tape. Repeat the process until you have the correct amount of tape to perfectly level your nozzles.
Lastly, remember that using the knobs underneath your bed will help in leveling the nozzle heights.