\begin{tikzpicture}

\tikzstyle{system} = [rectangle,very thick,minimum width=1.5cm,font=\scriptsize];
\tikzstyle{output} = [rectangle,very thick,rounded corners=3pt,minimum width=1.5cm,align=center,font=\scriptsize];

\begin{scope}[local bounding box=MULTIPLE]
    \node [system,draw=orange,text=orange] (engine3) at (0,0) {Engine $n$};
    \node [system,draw=ugreen,text=ugreen,anchor=south] (engine2) at ([yshift=0.6cm]engine3.north) {Engine $2$};
    \node [system,draw=red,text=red,anchor=south] (engine1) at ([yshift=0.3cm]engine2.north) {Engine $1$};

    \node [output,draw=orange,text=orange,anchor=west] (output3) at ([xshift=0.5cm]engine3.east) {Output $n$};
    \node [output,draw=ugreen,text=ugreen,anchor=west] (output2) at ([xshift=0.5cm]engine2.east) {Output $2$};
    \node [output,draw=red,text=red,anchor=west] (output1) at ([xshift=0.5cm]engine1.east) {Output $1$};

    \draw [very thick,decorate,decoration={brace}] ([xshift=3pt]output1.north east) to node [midway,name=final] {} ([xshift=3pt]output3.south east);

    \node [output,draw=ublue,text=ublue,minimum width=1cm,right=0pt of final,minimum height=2.5em] () {Final\\Output};

    \draw [->,very thick] (engine1) to (output1);
    \draw [->,very thick] (engine2) to (output2);
    \draw [->,very thick] (engine3) to (output3);

    \node [] () at ([yshift=0.4cm]output3.north) {$\vdots$};
\end{scope}

\begin{scope}[local bounding box=SINGLE]
    \node [output,draw=ugreen,text=ugreen,anchor=west] (output3) at ([xshift=4cm]output3.east) {Output $n$};
    \node [output,draw=ugreen,text=ugreen,anchor=west] (output2) at ([xshift=4cm]output2.east) {Output $2$};
    \node [output,draw=ugreen,text=ugreen,anchor=west] (output1) at ([xshift=4cm]output1.east) {Output $1$};

    \node [system,draw=ugreen,text=ugreen,anchor=east,align=center,inner sep=1.9pt] (engine) at ([xshift=-0.5cm]output2.west) {Single\\Engine};

    \draw [very thick,decorate,decoration={brace}] ([xshift=3pt]output1.north east) to node [midway,name=final] {} ([xshift=3pt]output3.south east);

    \node [output,draw=ublue,text=ublue,minimum width=1cm,right=0pt of final,minimum height=2.5em] () {Final\\Output};

    \draw [->,very thick] (engine.east) to (output1.west);
    \draw [->,very thick] (engine.east) to (output2.west);
    \draw [->,very thick] (engine.east) to (output3.west);

    \node [] () at ([yshift=0.4cm]output3.north) {$\vdots$};
\end{scope}

\node [align=center,anchor=north,font=\small] () at ([yshift=-0.3cm]MULTIPLE.south) {(a) combing outputs from\\multiple translation engines};
\node [align=center,anchor=north,font=\small] () at ([yshift=-0.3cm]SINGLE.south) {(b) combing outputs from a\\single translation engine};

\end{tikzpicture}