  We develop a methodology to optimise the measurement of Baryon Acoustic
Oscillation (BAO) from a given galaxy sample. In our previous work, we
demonstrated that one can measure BAO from tracers in under-dense regions
(voids). In this study, we combine the over-dense and under-dense tracers
(galaxies & voids) to obtain better constraints on the BAO scale. To this end,
we modify the de-wiggled BAO model with an additional parameter to describe
both the BAO peak and the underlying exclusion pattern of void 2PCFs. We show
that after applying BAO reconstruction to galaxies, the BAO peak scale of both
galaxies and voids are unbiased using the modified model. Furthermore, we use a
new 2PCF estimator for a multi-tracer analysis with galaxies and voids. In
simulations, the joint sample improves by about 10% the constraint for the
post-reconstruction BAO peak position compared to the result from galaxies
alone, which is equivalent to an enlargement of the survey volume by 20%.
Applying this method to the BOSS DR12 data, we have an 18% improvement for the
low-z sample (0.2<z<0.5), but a worse constraint for the high-z sample
(0.5<z<0.75), which is consistent with statistical fluctuations for the current
survey volume. Future larger samples will give more robust improvements due to
less statistical fluctuations.
