  Prior social contagion models consider the spread of either one contagion at
a time on interdependent networks or multiple contagions on single layer
networks or under assumptions of competition. We propose a new threshold model
for the diffusion of multiple contagions. Individuals are placed on a multiplex
network with a periodic lattice and a random-regular-graph layer. On these
population structures, we study the interface between two key aspects of the
diffusion process: the level of synergy between two contagions, and the rate at
which individuals become dormant after adoption. Dormancy is defined as a
looser form of immunity that models the ability to spread without resistance.
Monte Carlo simulations reveal lower synergy makes contagions more susceptible
to percolation, especially those that diffuse on lattices. Faster diffusion of
one contagion with dormancy probabilistically blocks the diffusion of the
other, in a way similar to ring vaccination. We show that within a band of
synergy, contagions on the lattices undergo bimodal or trimodal branching if
they are the slower diffusing contagion.
