Here is an example Ghostscript commandline, which would convert and merge the two (or more) PostScript files into one PDF in a one go:
gswin32c.exe ^
-o c:/path/to/output.pdf ^
-sDEVICE=pdfwrite ^
-dPDFSettings=/Screen ^
[...more desired parameters (optional)...] ^
/path/to/first.ps ^
/path/to/second.ps ^
/path/to/third.pdf
Edit: my first shot had falsely assumed PDF input files. It works of course with PostScript as well (or even a mix of PS/PDF)… And the output may also be PS.