  Cascading light through two thermal vapour cells has been shown to improve
the performance of atomic filters that aim to maximise peak transmission over a
minimised bandpass window. In this paper, we explore the atomic physics
responsible for the operation of the second cell, which is situated in a
transverse (Voigt) magnetic field and opens a narrow transmission window in an
optically thick atomic vapour. By assuming transitions with Gaussian line
shapes and magnetic fields sufficiently large to access the hyperfine
Paschen-Back regime, the window is modelled by resolving the two transitions
closest to line centre. We discuss the validity of this model and perform an
experiment which demonstrates the evolution of a naturally abundant Rb
transmission window as a function of magnetic field. The model results in a
significant reduction in two-cell parameter space, which we use to find
theoretical optimised cascaded line centre filters for Na, K, Rb and Cs across
both D lines. With the exception of Cs, these all have a better figure of merit
than comparable single cell filters in literature. Most noteworthy is a Rb-D2
filter which outputs >92% of light through a single peak at line centre, with
maximum transmission 0.71 and a width of 330 MHz at half maximum.
