[Tex/LaTex] Two `minipage` inside command

macrosminipage

I want two minipages side-by-side. I can do this easily. But then I want to turn it into a command, and the minipages don't line up any more, even thought the code looks identical.

problem

What is wrong? In the MWE i show what I want without the command, and then try to put it into command.

\documentclass{article}

\newcommand{\dualminipage}[2][]
{
\begin{minipage}[t][][t]{0.45\linewidth}
#1
\end{minipage}\hfill
\begin{minipage}[t][][t]{0.45\linewidth}
#2
\end{minipage}
}

\begin{document}
% Firstly the minipages put simply.
\begin{minipage}[t][][t]{0.45\linewidth}
This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. 
\end{minipage}\hfill
\begin{minipage}[t][][t]{0.45\linewidth}
This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. 
\end{minipage}

\vspace{0.5in}
% Now the minipages put into a command

\dualminipage {
This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. This is the left-hand side. 
}
{
This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. This is the right-hand side. 
}
\end{document}

Best Answer

Try

\newcommand{\dualminipage}[2]
{
\begin{minipage}[t]{0.45\linewidth}#1\end{minipage}
\hfill
\begin{minipage}[t]{0.45\linewidth}#2\end{minipage}
}

It seems that second square backed in your\dualminipage definition make a problem.

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