Multi color printing

I am planning to start with two color printing.
The instructions how to do the slicing seems clear.
But how can I design a multi object design which is needed for two color printing.
Before I buy the second nozzle I want to have an understanding how to use it.
I use 123D for my design work.

You need to ensure that you don’t combine (boolean join) objects in the design that you wish to be different colours. The software in the slicer has to be able to determine separate objects to be able to offer you the option of assigning one colour or another. For example, in 123D (I think, I use Fusion 360) you can add text to a surface and extrude it inwards (for example). Each letter should remain as a different object in the stl file unless you combine them during your design.

Basically you need separate meshes. They don’t need to sit in separate files. One stl can contain them all.
Two ways to build your objects :
§ first is to make an object. When finished, select vertices/edges and/or faces that will be common to both objects. Duplicate them but take car not to move them at all. And separate to make a new object. You can now start building your second mesh from the new duplicated vertices/edges and/or faces. As says pannekoek, faces should never cross each other.
§ second way is to build two separate objects and make a ‘difference’ (so called in blender) boolean operation.
Hope this will help.
If not clear don’t hesitate to ask.

Stay tuned… I plan to post a vid.