For printing flexible materials this mod might be helpfull: https://www.youmagine.com/designs/ultimaker-2-straight-tube-flexible-filament-spool-holder
For using dual head configuration I have seen something with magnets on this forum, this thread might be interesting: https://ultimaker.com/en/community/10657-a-different-multi-extrusion-approach-um-tool-printhead-changer
If you have an idea about extruding silicone with a syringe, please let it know, I also am interested