For two planes:
$(\vec{p} - \vec{p_0})^T \vec{n_p} = 0$
$(\vec{q} - \vec{q_0})^T \vec{n_q} = 0$
Prove if the normals are orthogonal, i.e. $\vec{n_p}^T \vec{n_0} = 0$, then all vectors (except potentially the vectors along the intersection of two planes) $\vec{p}-\vec{p_0}$ and $\vec{q} - \vec{q_0}$ are orthogonal.