We study the pairing of massive black holes embedded in a massive circum–nuclear, rotationally supported disc, until they form a close binary. Using high resolution SPH simulations, we follow the black hole dynamics, and in particular the eccentricity evolution, as a function of the composition in stars and gas of the disc. Binary–disc interaction always leads to orbital decay and, in case of co–rotating black holes, to orbit circularization. We present also a higher resolution simulation performed using the particle–splitting technique showing that the binary orbital decay is efficient down to a separation of ~ 0.1 pc, comparable to our new resolution limit. We detail the gaseous mass profile bound to each black hole. Double nuclear activity is expected to occur on an estimated timescale of ≲ 10 Myrs.