Here is a Dual Boot build of RetroPie & LibreELEC (16.1 Jarvis) for the Raspberry Pi 3:
As usual this is a NOOBS USB installer build so please follow the instructions in the included ‘readme’ file.
This build is the first to incorporate my new ‘smart’ switching scripts, which will seek out which partition is the correct one before mounting it and loading the other OS. This prevents the incorrect partition being mounted if you were to mix and match OS’s, which could leave the card in an ‘un-bootable’ state.
I have created an ‘Always boot to KODI’ entry in the RetroPie menu. Use this to always boot to KODI (obviously!) on every boot and you can turn it off again should you desire. More info on this in the readme file.
Other than that, both operating systems are standard so should work as they were designed. Please test it out and let me know your thoughts! 🙂
All the best.