After printing, make sure there are no plastics residuals in the filament paths (use a 2mm drill bit to clean the filament path if needed).
Ram a 4mm PTFE tube into the conical single output end. Use Pneufit M6x4mm connectors in the dual input end (example: https://www.123-3d.se/123-3D-Pneufit-rak-koppling-M6-x-4mm-i1623-t15033.html).
18 Feb 2024: Added a print profile with tighter fitting tolerance for the output PTFE tube.
21 Feb 2024: Added a version that does not require any Pneufit at all. Just friction between the y-splitter and the PTFE tubes. This is the version I'm currently using on my own machine.