  We forecast future constraints on local-type primordial non-Gaussianity
parameter $f_{\rm NL}$ with a photometric galaxy survey by Euclid, a continuum
galaxy survey by Square Kilometre Array (SKA), and their combination. We derive
a general expression for the covariance matrix of the power spectrum estimates
of multiple tracers to show how the so-called multitracer technique improves
constraints on $f_{\rm NL}$. In particular we clarify the role of the overlap
fraction of multiple tracers and the division method of the tracers. Our Fisher
matrix analysis indicates that stringent constraints of $\sigma (f_{\rm
NL})\lesssim 1$ can be obtained even with a single survey, assuming five mass
bins. When Euclid and SKA phase 1 (2) are combined, constraints on $f_{\rm NL}$
are improved to $\sigma (f_{\rm NL})= 0.61~(0.50)$.
