3D print on top of a model already adhered to the bed

This is a problem that has multiple solutions. Here’s 1 option for Luban:

  • Make the final model you want into a single model and slice as if it would be printed with a single filament non-stop
  • Find out the layer numbers where you want the filament to change in Luban.
  • Manually insert the gcode M600 before the layer where you want the change. This will pause and allow you to change the filament before continuing.

Another option would be to use Cura to slice your models with, and then use the plugin called PauseAtHeight to trigger something at the Z height you want

https://stefdev49.github.io/blog/cura/how-to-insert-M600-with-cura

If you really want to do this with separate models there’s ways, but they get even more complicated and I would probably recommend against it.