jmc

Let $P$ and $Q$ be the the intersection points of the two parabolas. Then by definition of the parabola, the distance from $P$ to their common focus $F$ is equal to the distance from $P$ to the $x$-axis. Also, the distance between $P$ to $F$ is equal to $P$ to the $y$-axis. This means $P$ is equidistant to both the $x$-axis and $y$-axis, so $P$ must lie on the line $y=-x.$



By the same argument, $Q$ also lies on the line $y=-x.$ Therefore, the slope of $\overline{PQ}$ is $\boxed{-1}.$