  The coalescence of two supermassive black holes (SMBHs) produces powerful
gravitational-wave (GW) radiation and, if gas is present in the vicinity, also
an electromagnetic (EM) counterpart. In the standard picture, an EM outburst
will be produced when the binary "decouples" from the circum-binary disc and
starts "squeezing" the disc inside the secondary orbit, resulting in its quick
accretion on to the primary black hole. Here we use analytical arguments and
numerical simulations to show that the disc within about $20~R_S$ of a SMBH
survives the merger without being depleted. The reason is a "second
decoupling": the inner disc thickens due to tidal heating and inefficient
cooling, effectively decoupling from the interaction of the binary. We show
that this second decoupling quenches the heating sources in the disc ${\cal
O}(10^2)$ days before coalescence. This will render the peak UV/X-ray
luminosity significantly weaker than previously thought. After the merger, the
residual disc cools down and expands, merging with the outer disc rather than
being completely accreted. This results in continuous EM emission, hindering
the detection of the cut-off and re-brightening proposed in earlier studies.
