  We present numerical and analytical results for the optical and acoustic
plasmon collective modes of coupled massless-Dirac two-dimensional electron
systems. Our results apply to topological insulator (TI) thin films and to two
graphene sheets separated by a thin dielectric barrier layer. We find that
because of strong bulk dielectric screening TI acoustic modes are locked to the
top of the particle-hole continuum and therefore probably unobservable.
