I use this to join two PTFE tubes before entering the toolhead extruder:
MMU --→ 3mm ID PTFE --→ Connector (this model) --→ 2.5mm ID PTFE --→ Toolhead Extruder
I chamfered the 2.5mm ID PTFE hard so that filament moves smoothly across the joint point
The author remixed this model.
I remodeled https://www.printables.com/model/268911-ptfe-tube-connector-with-pc4-01-pc4-m10-fitting in Fusion 360. I enlarged the inner diameter of the tube between the two connectors to 5mm, so that two PTFE tubes can reach out to each other in the middle.