A heated hotend expands and it might happen that gaps occur where melted filament could be accumulated. Hot tightening prevents that and is done using the following steps: Heat up to maximum temperature wait and then apply value a torque to the nozzle. For ED3 Titan aero the max temperature is 300°C and the torque 2Nm.

Mounting two extruder allows to print with two different materials and is used for printing support material with water-soluble material.

The two nozzles will not have the exact hight over the bed, there will always be a fraction of some micrometers. it might happen that the nozzle closer to the bed scratches away what the other nozzle has printed. The nozzle closer to the bed should therefore be used as first extruder to print the first and regular material. The second nozzle will then print the second an mostly the optional material. It will print the maybe water-soluble material on a slightly higher gap and therefore print quality will be less.

Using gaps in the mounting screws can be used to adjust the height differences. Also not perfectly mounted vertical extruder get a height difference.

Double extruder reduce the maximum printable dimension of one axis. If just a slave nozzle is added then and first nozzle is placed as a single extruder no reduction occurs if just one material is printed.