  We present a $\mathbf{k} \cdot \mathbf{p}$ theory of photonic crystals
containing gain and loss in which the gain and loss are added to separate
primitive cells of the underlying Hermitian system, thereby creating a
supercell photonic crystal. We show that the supercell bands of this system can
merge outward from the degenerate contour formed from folding the bands of the
underlying Hermitian system into the supercell Brillouin zone, but that other
accidental degeneracies in the band structure of the underlying Hermitian
system do not yield band merging behavior. Finally, we show that the modal
coupling matrix in PhCs with balanced gain and loss is trace-less, and thus the
imaginary components of the eigenvalues can only move relative to one another
as the strength of the gain and loss is varied, without any collective motion.
