documentclass{article}
usepackage{multirow}
% ---------------
begin{document}
begin{tabular}{l l}
some & text \
& b \[-4pt]
multirow{-2}{*}{a}
& b \
some & text \
end{tabular}
% ----------------
begin{tabular}{l l}
some & text \
& b \
multirow{-2}{*}{a}
& b \
some & text \
end{tabular}
end{document}