  We quantify the resources required for entangling two uncoupled spin qubits
through an intermediate mesoscopic spin system (MSS) by indirect joint
measurement. Indirect joint measurement benefits from coherent magnification of
the target qubits' state in the collective magnetization of the MSS; such that
a low-resolution collective measurement on the MSS suffices to prepare
post-selected entanglement on the target qubits. A MSS consisting of two
non-interacting halves, each coupled to one of the target qubits is identified
as a geometry that allows implementing the magnification process with
experimentally available control tools. It is proved that the requirements on
the amplified state of the target qubits and the MSS perfectly map to the
specifications of micro-macro entanglement between each target qubit and its
nearby half of the MSS. In the light of this equivalence, the effects of
experimental imperfections are explored; in particular, bipartite entanglement
between the target qubits is shown to be robust to imperfect preparation of the
MSS. Our study provides a new approach for using an intermediate spin system
for connecting separated qubits. It also opens a new path in exploring
entanglement between microscopic and mesoscopic spin systems.
