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.