two equation one number

\begin{equation}\label{Ionx}

\begin{split}

x_I=\dfrac{\partial x}{\partial I}=\dfrac{t'(I)\pi^{*}_{x^*x^*}}{D}>0 \\

x^{*}_{I}=\dfrac{\partial x^*}{\partial I}=-\dfrac{t'(I)\pi^{*}_{x^*x}}{D}<0

\end{split}

\end{equation}