https://www.instructables.com/HeaphoneStero-Splitter/
Heaphone/Stero Splitter : 4 Steps - Instructables
Heaphone/Stero Splitter: Here's a very simple project for a rainy afternoon. You my already have all the parts, as I did, and even if not, the cost is minimal...
heaphonesterosplitterstepsinstructables