  Galaxy surveys contain information on the largest scales via wide-angle and
relativistic contributions. By combining two different galaxy populations, we
can suppress the strong cosmic variance on ultra-large scales and thus enhance
the detectability of the signals. The relativistic Doppler and Sachs-Wolfe
effects are of a similar magnitude to the leading wide-angle corrections, so
that it is important to treat them together, especially since they can
partially cancel. The power spectra depend on the choice of line of sight for
each galaxy pair and we present results for a general line of sight. Then we
estimate the detection significance of the auto- and cross-power spectra for a
variety of cases. We use two futuristic galaxy samples based on a `beyond-DESI'
survey and a SKA Phase 2 survey, covering 15,000\,deg$^2$ up to $z=1$. We find
a detection significance for the total relativistic wide-angle effects that
ranges from $\sim 5\sigma$ to $>15\sigma$, depending on the line-of-sight
configuration.
