documentclass[tikz]{standalone}
usetikzlibrary{positioning,fit}

begin{document}

begin{tikzpicture}
node (a) [draw=black] {a};
node (b) [draw=black,on grid,below=of a] {long text};
node (p) [draw=black,fit={(a) (b)}] {};

node (c) [draw=red] {c};
node (d) [draw=red,below=of c] {d};
node (q) [right=of p,draw=red,fit={(c) (d)}] {};
end{tikzpicture}
end{document}

documentclass[tikz,margin=3mm]{standalone}
usetikzlibrary{fit, positioning}

begin{document}

begin{tikzpicture}[
node distance = 8mm and 4 mm,
every node/.style = {inner sep=1mm, minimum height=1.5em}
                    ]
node (a) [draw=black] {a};
node (b) [draw=black,below=of a] {long text};
node (p) [draw=black,fit={(a) (b)}] {};

node (c) [draw=red,right=of a |- b] {c};
node (d) [draw=red,below=of c] {d};
node (q) [draw=red,fit={(c) (d)}] {};%right=of p,
end{tikzpicture}
end{document}