bearing a particle of species A. Similarly, birth of $B$ individuals \[i.e., the transitions $(0,0) \to (0,1)$ and $(1,0) \to (1,1)$\], occurs at rate $\lambda_B r_B$, with $r_B$ the fraction of NNs bearing a particle of species B. Death at singly occupied sites, $(1,0) \to (0,0)$ and $(0,1) \to (0,0)$, occurs at a rate of unity, as in the basic CP. The transitions $(1,1) \to (1,0)$ and $(1,1) \to (0,1)$, corresponding to death at a doubly occupied site, occur at rate $\mu$. The set of transition rates defined above describes a pair of contact processes inhabiting the same lattice. If $\mu=1$ the two processes evolve independently, but for $\mu < 1$ they interact [*symbiotically*]{} since the annihilation rates are reduced at sites with both species present.

The phase diagram of the 2SCP exhibits four phases: (i) the fully active phase with nonzero populations of both species; (ii) a partly active phase with only $A$ species; (iii) a partly active phase with only $B$ species; (iv) the inactive phase in which both species are extinct. The latter is absorbing while the partly active phases represent absorbing subspaces of the dynamics. Extensive simulations on rings and on the square lattice indicate that 