[@okuma] have heightened interest in such transitions. In this context, in [@scp] we employed extensive simulations and field-theoretical arguments to show that the critical scaling of the 2SCP is consistent with that of directed percolation (DP), which is known to describe the basic CP [@note1], and is generic for absorbing-state phase transitions [@janssen; @grassberger].

In this work we examine some of the issues regarding the 2SCP left open in the original study [@scp]: (1) Can mean-field predictions be improved on? (2) What is the phase boundary for unequal creation rates? (3) Does the model exhibit a discontinuous phase transition in two dimensions, for strong symbiosis, or in the presence of diffusion?

The mean-field theory for the 2SCP [@scp], at both one- and two-site levels, predicts a discontinuous phase transition for strong symbiosis in any number of dimensions. Discontinuous phase transitions to an absorbing state are not possible, however, in one-dimensional systems with short-range interactions and free of boundary fields [@hinrichsen]. We have indeed verified this general principle in simulations of the one-dimensional model. The simulations reported in [@scp] did not reveal a discontinuous transition in two dimensions ($d=2$) either. In the present work we aim to provide a better theoretical 