  We develop theory for a two-component miscible dipolar condensate in a planar
trap. Using numerical solutions and a variational theory we solve for the
excitation spectrum and identify regimes where density- and spin-roton
excitations are favored. We characterize the various instabilities that can
emerge in this system over a wide parameter regime and present results for the
stability phase diagram. Importantly this allows us to identify the parameter
regimes where a novel roton-immiscibility transition can occur, driven by the
softening of the spin roton excitation.

Quantum Gases