insertsectionhead
  ifxinsertsubsectionhead@emptyrelaxelse$quadmidquad$insertsubsectionheadfi

documentclass{beamer}

makeatletter
setbeamertemplate{footline}{%
leavevmode
vbox{begin{beamercolorbox}[dp=1.25ex,ht=2.75ex]{fg=black}%
  hspace*{1em}insertsectionhead%
  ifxinsertsubsectionhead@emptyrelaxelse$quadmidquad$insertsubsectionheadfi
  end{beamercolorbox}%
  }%
}
makeatother

begin{document}

section{Test section One}
begin{frame}
test
end{frame}
subsection{Test subsection One One}
begin{frame}
test
end{frame}
subsection{Test subsection One Two}
begin{frame}
test
end{frame}

end{document}