Many printers come with the ability to use more than one extruder during the print. This can allow prints in multi-color, or even dissolvable support material. While this sounds like a great addition to FDM printing, there are many headaches and problems involved.
Leveling the Nozzles
Leveling the nozzles can be one of the most difficult things involved with dual extruding. Each printer has its own method of leveling, and some can seem impossible to perfect. Larger nozzled printers with larger layer heights are a bit easier, but a slight difference in their heights will lead to the lower nozzel knocking into the print. Some printers even require the entire hotend to be disassembled, the nozzles leveled, and then having the hotend reassembled before printing can be successfully attempted.
Dissolvable Support Material
When dual extruding it is often suggested to use High Impact Polystyrene (HIPS) for support material. This is because HIPS is dissolvable in Limonene, a colorless liquid hydrocarbon. While we have experienced some difficulty with dissolvable HIPS, the idea is that you can increase your support structure fill amount up to 100. This should remove all ugly undersides of prints as well as make post-processing much easier. You can also print support in PVA (which is dissolvable in water), or in PLA (which can be dissolved in Sodium Hydroxide).
Having dual or multiple extrusions also allows for multi-colored prints. An object can be designed with multiple colors or there are designs already available for download online.
Generally, when printing with dual extrusions, priming towers and ooze shields are required in order to have a print with a good surface quality.