In this work we analyse by means of first principles electronic structure calculations the issue of the interactions between dopants in N and P doped SiC, and the implications for co-doping in SiC. Despite its potential high attractiveness, experimental measurements proved that co-doping increase the activation rate only in a limited range of implanted doses. We speculate on a possible explanation of the better high-dose activation rate of plain P doping with respect to N and P co-doping and plain N doping.