How do I create a lip and groove to two separate parts?

I created a solid body, split it, and then saved the two parts individually. I later learned of lip and groove feature but can't find how to apply it to the two parts.

I tried to create an assembly from the two parts and save it as a new part, but the option is not available for the part. The option is also not available as an assembly feature.

Thank you for any guidance you can provide!