gs -q -dNOPAUSE -dBATCH -sDEVICE=pdfwrite -sOutputFile=combined.pdf first-input.pdf second-input.pdf

There's a GUI tool, called Combine PDF, which purports to do the same thing.  But the output loses text searchability.  Whereas this method preserves searchability.

NOTE: you may need to do 'sudo port install ghostscript' first

Combine PDFs with Ghostscript

Bash