\documentstyle{article}
%Reported by C. Spiel, @{...} format should close previous cell,
%also when a P element is nested inside TD.
\begin{document}
\begin{tabular}{p{0.25\linewidth}@{\qquad}l}
  Head1  &  Head2  \\
  \relax% macro call plus following blank lines[s] screw up the output.

  Data11  &  Data12  \\
  Data21  &  Data22  \\
\end{tabular}
\end{document}
