Superscript and subscript together in text mode

Duplicate the definition of \textsuperscript:

\documentclass{article}

\makeatletter
\DeclareRobustCommand{\textsupsub}[2]{{%
  \m@th\ensuremath{%
    ^{\mbox{\fontsize\sf@size\z@#1}}%
    _{\mbox{\fontsize\sf@size\z@#2}}%
  }%
}}
\makeatother

\begin{document}

X\textsupsub{ab}{c}X\textsupsub{c}{ab}X

\end{document}

Add \strut to the second argument if you prefer.