sum_{i in X} sum_{i in bar{X}}

documentclass{article}
usepackage{mathtools}
begin{document}
[
  adjustlimitssum_{i in X}sum_{i in bar{X}}
]
end{document}

documentclass{article}
begin{document}
[
  sum_{i in X}sum_{i in bar{X}} quad mbox{compared to} quad
  sum_{i in Xvphantom{bar{X}}}sum_{i in bar{X}}
]
end{document}end{document}​