<p>Is it OK (or even recommended/good practice) to <code>#include</code> a <code>.c</code> file in another <code>.c</code> file?</p>

No.
Depending on your build environment (you don't specify), you may find that it works in exactly the way that you want.
However, there are many environments (both IDEs and a lot of hand crafted Makefiles) that expect to compile *.c - if that happens you will probably end up with linker errors due to duplicate symbols.
As a rule this practice should be avoided.
If you absolutely must #include source (and generally it should be avoided), use a different file suffix for the file.