functions in x.prg can call functions in y.prg, not by how they are compiled, but by linking.
If you would care to post your .lnk file on here, we can take a look and suggest alternatives, or, generally speaking,
each file (obj) must be referenced in your linking file (lnk) either as a file entry....
file x
file y
or as an overlayed file...
file x
beginarea
file y
endarea
hope this helps