  The fluctuations generated by Inflation are nearly Gaussian in the simplest
models, but may be non-Gaussian in more complex models, potentially leading to
signatures in the late Universe. In particular, local type primordial
non-Gaussianity induces scale-dependent bias in tracers of the matter
distribution. This non-Gaussian imprint in the tracer power spectrum survives
at late times on ultra-large scales where nonlinearity is negligible. In order
to combat the problem of growing cosmic variance on these scales, we use a
multi-tracer analysis that combines different tracers to maximise any
primordial signal. Previous work has investigated the combination of a
spectroscopic galaxy survey with a 21cm intensity mapping survey in single-dish
mode. We extend this work by considering instead the case where the 21cm
intensity mapping survey is optimised for interferometer mode. As examples, we
use two multi-tracer pairs of surveys: one at high redshift ($1 \le z \le 2$)
and one at very high redshift ($2 \le z \le 5$). The 21cm surveys are idealised
surveys based on HIRAX and PUMA. We implement foreground-avoidance filters and
use detailed models of the interferometer thermal noise. The galaxy surveys are
idealised surveys based on Euclid and MegaMapper. Via a simple Fisher forecast,
we illustrate the potential of the multi-tracer. Our results show a $\sim 20 -
30\%$ improvement in precision on local primordial non-Gaussianity from the
multi-tracer. Furthermore, we investigate the effects on constraints of varying
the parameter of non-Gaussian galaxy assembly bias and of varying the
parameters of the intensity mapping foreground filters. We find that the
non-Gaussian galaxy assembly bias parameter causes a greater change in the
constraints on local primordial non-Gaussianity than the foreground filter
parameters.
