  The optimal quantum measurements for estimating individual parameters might
be incompatible with each other so that they cannot be jointly performed. The
tradeoff between the estimation precision for different parameters can be
characterized by information regret -- the difference between the Fisher
information and its quantum limit. We show that the information-regret-tradeoff
relation can give us not only an intuitive picture of the potential in
improving the joint scheme of estimating the centroid and the separation, but
also some clues to the optimal measurements for the sequential scheme. In
particular, we show that, for two incoherent point sources with a very small
separation, the optimal measurement for the separation must extract little
information about the centroid, and vice versa.
