Synergies between intensity maps of hydrogen lines
  We study synergies between HI 21cm and H$\alpha$ intensity map observations,
focusing on SKA1-like and SPHEREx-like surveys. We forecast how well such a
combination can measure features in the angular power spectrum on the largest
scales, that arise from primordial non-Gaussianity and from general
relativistic effects. For the first time we consider Doppler, Sachs-Wolfe and
integrated SW effects separately. We confirm that the single-tracer surveys on
their own cannot detect general relativistic effects and can constrain the
non-Gaussianity parameter $f_{\rm NL}$ only slightly better than Planck. Using
the multi-tracer technique, constraints on $f_{\rm NL}$ can be pushed down to
$\sim1$. Amongst the general relativistic effects, the Doppler term is
detectable with the multi-tracer. The Sachs-Wolfe terms and the integrated SW
effect are still not detectable.
