The Gaia DR2 catalogue is formally independent from Gaia DR1. Although there is a significant overlap in terms of the input observations used to construct them (as described in the rest of the documentation). This means a direct one-to-one relationship between the source identifiers from DR1 and DR2 is not guaranteed. That is, the same physical object might have different source_id in each release. Having the same numerical identifier in both catalogues is also not complete assurance they refer to the same astronomical object in both releases. Further details can be found in Section 2.4.9, including source merging, splitting and high proper motion star treatment.
This means deciding which sources in different data releases might be physically related is a scientific problem on its own, where all the evidence at hand must be carefully considered. In this sense, a best neighbours table has not been produced. Only the neighbourhood has been characterised using table dr1_neighbourhood. It contains all possible matches between DR1 and DR2 sources following the criteria described below.