  Vapour cell spectroscopy is an essential technique in many fields; in
particular, nearly all atom and ion trapping experiments rely on simultaneous
spectroscopy of two atomic transitions, traditionally employing separate
apparatus for each transition. Here, we demonstrate simultaneous spectroscopy
on two atomic transitions using spatially-overlapped beams from two independent
lasers, within a single spectroscopic apparatus. We show that, in addition to
aiding compactness, this approach offers superior performance, leading to
sharper spectroscopy peaks and stronger absorption signals. Doppler-free
locking features become visible over a frequency range several hundred MHz
wider than for standard saturated absorption spectroscopy. By exploring the
full, 2D parameter space associated with dual-frequency spectroscopy, we reveal
a lattice-like structure of sharp resonance features in 2D frequency space,
which enhances experimental versatility by allowing laser frequency
stabilisation anywhere within a wide manifold of locations in 2D frequency
space. The process of simultaneous frequency stabilisation of two lasers is
analysed in detail, revealing that it can be expected to produce significant
improvements in laser stability when compared to conventional, single-frequency
spectroscopy.
