  Upcoming large-scale structure surveys will be able to measure new features
in the galaxy two point correlation function. Relativistic effects appear on
large scales as subtle corrections to redshift-space distortions, showing up as
a dipole and octupole when cross-correlating two different tracers of dark
matter. The dipole and octupole are very sensitive to the evolution and
magnification biases of the observed tracers which are hard to model accurately
as they depend upon the derivative of the luminosity function at the flux limit
of the survey. We show that splitting a galaxy population into bright and faint
samples allows us to cross-correlate these and constrain both the evolution
bias and magnification bias of the two samples -- using the relativistic odd
multipoles of the correlation function, together with the even Newtonian
multipoles. Although the octupole has much lower signal-to-noise than the
dipole, it significantly improves the constraints by breaking parameter
degeneracies. We illustrate this in the case of a futuristic survey with the
Square Kilometre Array, and demonstrate how splitting the samples in different
ways can help improve constraints. This method is quite general and can be used
on different types of tracers to improve knowledge of their luminosity
functions. Furthermore, the signal-to-noise of the dipole and octupole peaks on
intermediate scales, which means that that they can deliver a clean measurement
of the magnification bias and evolution bias without contamination from local
primordial non-Gaussianities or from systematics on very large scales.
