Files for two colour printing - want simple examples

I'm not sure what you're referring to here. Color 3d printing requires that your model have either vertex color information or textures with color information. If you plan to use OBJ for color printing, then you will need a zip file which includes the OBJ of the model, the .mtl file which contains the model's texture information, and finally the image texture files in JPG or PNG format. You can also use .WRL or .VRML file types for your model, since those are capable of holding 3d model vertex color information instead of image textures.

STL file format does not contain color information for your printed files. There are ways around this, but it's easier to just work with OBJ or VRML instead.

Can I not have just two models which are to be printed with two extruders? I created two OBJ files, and could load both of them in Ultimaker Cura, and even get gcode from it, but I doubt that will work. I have given no information on material or texture or colour.

I understand this, but is this what I need with a dual extruder printer also, so that the two parts in two colours are printed together at the same time?

I think the same process would also apply to a dual extruder. The only difference is that the boolean objects can be printed together instead of separately. In other words, after you boolean the plus sign from the brick, you can place the two models together in the slicer as you intend them to be printed. It depends on how the slicer works and if you want the two parts to be removable or not.