  We analyse the conditional states in which one part of a split spin-squeezed
state is left, upon performing a collective spin measurement on the other part.
For appropriate measurement directions and outcomes, we see the possibility of
obtaining states with high quantum Fisher information, even reaching the
Heisenberg limit. This allows us to propose a metrological protocol that can
outperform standard approaches, for example in a situation where the number of
particles in the probe is bounded. The robustness of this protocol is
investigated by considering realistic forms of noise present in cold-atom
experiments, such as particle number fluctuations and imperfect detection.
Ultimately, we show how this measurement-based state preparation approach can
allow for the conditional (\ie heralded) preparation of spin Schr\"{o}dinger's
cat states even when the initial state before splitting is only mildly
squeezed.
