\documentclass{article}
\usepackage{pgfplots}
\begin{document}
\makeatletter

\begin{tikzpicture}
\begin{axis}
\addplot[blue, quiver={u=1,v=3*x}, -stealth,samples=15] {x^3};
\addlegendentry{test}

\addplot[mark=*,red, quiver={u=1,v=2*x}, -stealth,samples=15] {x^2};
\label{test}
\end{axis}
\end{tikzpicture}

Second legend:
\ref{test}
\end{document}

