Create multiple arrows between items in displaymath

Question

I want to draw a diagram with the following two implications:

NM-CCA1 =/=> NM-CCA2
                 <=

But so far I do not know how to draw two arrows between the items. I can only draw one and I also cannot explicitly show that one implication does not hold. How would I go about this?

\begin{displaymath}
\xymatrix{NM-CCA1 \ar@2{->}[r] & NM-CCA2 \ar@2{->}[l]}
\end{displaymath}

Answer 1

Here, with a stack.

\documentclass{article}
\usepackage{stackengine}
\stackMath
\begin{document}
\[
\def\stacktype{L}
\textrm{NM--CCA1} 
\mathrel{\stackunder[.7\baselineskip]{\stackon[0pt]{\Longrightarrow}{/}}{\Longleftarrow}} 
\textrm{NM--CCA2}
\]
\end{document}

or maybe this, with a one-macro change:

\documentclass{article}
\usepackage{stackengine}
\stackMath
\begin{document}
\[
\def\stacktype{L}
\textrm{NM--CCA1} 
\mathrel{\stackanchor[.7\baselineskip]{\stackon[0pt]{\Longrightarrow}{/}}{\Longleftarrow}} 
\textrm{NM--CCA2}
\]
\end{document}