Making 2 parts into 1 NOT an Assembly

in one part, go to insert -> part and select the other part. if you want to bring the features in, make sure you select the option to "break link" to existing file.

Alternatively, if you're not familiar with working with surfaces you can thicken them into solids again.

Just select one of the bodies on the feature tree and then use the "Thicken" tool and tick the box to try to create a solid out of the surface. At that point you can also choose whether you want to merge the entities or keep it as a multibody part.

JS,

Do you mind me asking why you don't want to make an Assembly of these two Parts?  And it you did, why would you need to save the Assembly as a Part?

When saving an assembly to a part, in the save dialog you get the option to save as Exterior faces (surfaces)/Exterior Components(Bodies/All Components(Bodies). This would achieve what you want but is of no advantage compared to simply inserting one part into the other as a derived part in this case.

Are you able to ZIP and post the assembly and parts?

I created a joined part with zero thickness but did not get any errors.