Numbering equations in dcases without \usepackage{empheq}

At the moment, I don't remember how to number the equations inside dcases , with the amsart class, without empheq, in order to assign a label to each one.

\documentclass{amsart}\usepackage{mathtools}\begin{document}\begin{subequations}\begin{equation}\begin{dcases}0 \le \frac{u+v}{2} \le 1 \\0 \le \frac{v-u}{2} \le 1 - \frac{u+v}{2}\end{dcases}\iff\begin{dcases}0 \le u+v \le 2 \\ \label{Psojjn}0 \le v-u \le 2 - (u+v)\end{dcases}\end{equation}\end{subequations}\end{document}

Addendum: My desidered output.