
\documentclass{article}
\usepackage[body={8cm,8cm}]{geometry}
\usepackage{lmodern}
\usepackage{mathtools}
\pagestyle{empty}
\begin{document}
\[
S_I = \sum_{%
    \begin{subarray}{l}
      (i,j) \in I^2 \\
      i \neq j}} 
    \end{subarray}
    }
  x_i + x_j
\]
\end{document}
