It's strange that you have a pdb of an homodimer with the only one chain. the problem is that if you need a pdb of the second chain you must have the crystallographic coordinates. Are you sure that the determined structure contains only one chains? Have you read the reference?
Typically the other symmetric chain is defined by matrix. You can try to download PDB from NCBI MMDB. It usually output the coordinates of symmetric chains as well, e.g., change “format” to “PDB” and click “download” at https://www.ncbi.nlm.nih.gov/Structure/pdb/2POR.