  Next-generation cosmological surveys will probe ever larger volumes of the
Universe, including the largest scales, near and beyond the horizon. On these
scales, the galaxy power spectrum carries signatures of local primordial
non-Gaussianity (PNG) and horizon-scale general relativistic (GR) effects.
However, cosmic variance limits the detection of horizon-scale effects.
Combining different surveys via the multi-tracer method allows us to reduce the
effect down cosmic variance. This method benefits from large bias differences
between two tracers of the underlying dark matter distribution, which suggests
a multi-wavelength combination of large volume surveys that are planned on a
similar timescale. We show that the combination of two contemporaneous surveys,
a large neutral hydrogen intensity mapping survey in SKA Phase1 and a
Euclid-like photometric survey, will provide unprecedented constraints on PNG
as well as detection of the GR effects. We forecast that the error on local PNG
will break through the cosmic variance limit on cosmic microwave background
surveys and achieve $\sigma(f_{NL})\simeq1.4-0.5$, depending on assumed priors,
bias, and sky coverage. GR effects are more robust to changes in the assumed
fiducial model, and we forecast that they can be detected with a
signal-to-noise of about $14$.
