In reality I am finding my self just using the for having easy access to the two colors of ABS that I might have loaded. Printing with both nozzles simultaneously is challenging! Since this is the first post on the subject I will talk about some of the teething problems that I had and later about printing simultaneously.
Selecting the right base profile
I use Simplify3D and that caused me a couple of problems. The first might be obvious but it was not to me. There are four stock profiles available in Simplify3D for the Wanhao (single extruder, dual extruder left, dual extruder right, and dual extruder both). The dual extruder left and dual extruder right only light up the left or right extruder while the dual one lets both be lit up. At least I think this is what goes on behind the scenes. I don't really understand the nuances of why there needs to be three profiles as temperature control for the extruders is granular. But in any case, I use the profiles as they are named.
Adjustments to be able to use the left/secondary extruder
Simplify3D is a good piece of software. It does a lot of things that the free alternatives don't do including more intelligent support structures, what I think is a faster print from the slicer, ability to print multiple profiles at the same time, and a good user interface to tweek a wide range of printer parameters. It is, however, one of the BUGGIEST pieces of software that I use. One of those bugs concerns how you need to configure to use the left nozzle assuming that you have selected the left profile as discussed above!
The problem is that anytime Simplify3D changes temperature on the secondary nozzle, or the build platform, the firmware needs to be reminded that we are using the secondary nozzle (assuming that is the case). If you do not do this then you will be in for a surprise as the primary extruder will start clicking as it has not been heated up and the printer will be directing a cold nozzle at your print instead of a nice hot one with plastic coming out of it!
It is pretty easy to correct this problem. Two changes are needed. First under the "Tools" menu select "Firmware Configuration". Get rid of the "T0" at the end of the "Set Bed Temperature" field. This will keep things from going south if you adjust the Build Platform Temperature. Second go to the "Scripts" tab of the active "Process". Select the tab for "Tool Change G-Code" and enter the following code: "M135 T1". This keeps the printer working on the secondary extruder.
Note that the above steps are not needed if you are running under the dual extruder profile or with the primary/right extruder.
Adjusting the nozzles so they are level
Obviously the two nozzles need to be perfectly level in order to print in two materials.
One way to make minor adjustments is to use a small piece of Klapton Tape as a shim. Do this by taking the extruder assembly out of it's carriage. The tape will go over the recess area where the bolt secures the extruder assembly to the carriage. Tape on the side where you want to raise the nozzle!
The other way of adjustment is more likely to be needed if you have had the extruder assembly in bits and pieces for some reason. In this case there are some printed things that can help:
thing:299331 (one example, there are more).
BTW, if you do have your extruder assembly in bits and pieces there are some good diagrams in this thread that show what is going on with the nozzles and heaters.
Keep the tips of the nozzles clean
If you switch back and forth between the two nozzles you need to remember to clean the nozzle that is not in use! I find that there will often be a little pit of filament still attached to the nozzle that is not in use. This little bit of filament will grab the stuff you are printing and make you want to kick the dog. Clean the nozzle before use and the SPCA will not need to come for you.