\documentclass[twoside]{article}
\usepackage{lipsum}
\usepackage{fancyhdr}
\pagestyle{fancy}
\let\Sectionmark\sectionmark
\def\sectionmark#1{\def\Sectionname{#1}\Sectionmark{#1}}
\let\Subsectionmark\subsectionmark
\def\subsectionmark#1{\def\Subsectionname{#1}\Subsectionmark{#1}}
\fancyhf{}
\fancyhead[CE]{\Sectionname}
\fancyhead[CO]{\Subsectionname}
\begin{document}
    \section{Section 1}
    \lipsum[1-2]
    \subsection{Subsection 1.1}
    \lipsum[1-18]
    \section{Section 2}
    \subsectionmark{}
    \lipsum[1-4]\\
    Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Ut purus elit, vestibu- lum ut, placerat ac, adipiscing vitae, felis. Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, vulputate a, magna. Donec vehicula augue eu neque. Pellentesque habitant morbi tristique senectus et ne- tus et malesuada fames ac turpis egestas. Mauris ut leo. Cras viverra metus rhoncus sem. Nulla et lectus vestibulum urna fringilla ultrices. Phasellus eu tellus sit amet tortor gravida placerat. Integer sapien est, iaculis in, pretium quis, viverra ac, nunc. Praesent eget sem vel leo.
    \subsection{Subsection 2.1}
    \lipsum
\end{document}