How should I properlly draw on a schematic a shunt going through the center of a toroid coil?
Draw it as a transformer where one of the windings has got a single turn.


T301 and T302 are the cores with sensing windings.  W303 is the shunt.

This schematic should translate well into the PCB layout.  The toroids aren't rigidly connected attached to each-other, or to the shunt.  So, in this schematic they are separate components.  If you need even more flexibility during layout, the shunt could be represented by two separate pads, instead of one component with two pads.