  While supermassive binary black holes inspiral toward merger they may also
experience significant accretion of matter from a surrounding disk. We study
the dynamics of this system, simultaneously describing the evolving spacetime
and magnetized plasma, and present the first relativistic calculation
simulating two equal-mass, non-spinning black holes as they inspiral from a
$20M$ ($G=c=1$) initial separation almost to merger, $\simeq 9M$ ($M$=binary
mass). Our dynamical results imply important observational consequences: for
instance, the accretion rate $\dot M$ onto the black holes first decreases and
then reaches a plateau, dropping by only a factor of $\sim 3$ despite the rapid
inspiral. An estimated bolometric light curve thus suggests some merging SMBBHs
may be quite luminous past the predicted decoupling from the circumbinary disk.
The minidisks through which the accretion reaches the black holes are very
non-standard: Reynolds, not Maxwell, stresses dominate, and they oscillate
between two states. In one part of the cycle, ``sloshing" streams transfer mass
from one minidisk to the other through the L1 point at a rate $\sim 0.1\times$
the accretion rate, carrying kinetic energy at a rate that can be as large as
the peak minidisk bolometric luminosity. We also discover that episodic
accretion drives minidisks with time-varying tilts. The unsigned poloidal
magnetic flux on the black hole event horizon is roughly constant at a
dimensionless level $\phi\sim 2-3$, but doubles just before merger; if the
black holes had significant spin, this flux indicates the potential for
powerful jets with variability driven by binary dynamics, another prediction of
potentially unique EM signatures. This simulation is the first to employ our
multipatch infrastructure \pwmhd, decreasing computational expense to $\sim
3\%$ of conventional single-grid methods' cost.
