Flow Update is meant to be used in combination with Birth Flow. It tracks IDs assigned from Birth Flow to know which particle is which. Perhaps in the future I can add a setting to match default IDs so you could achieve what you want (using Flow Update without emitting the particle using Birth Flow), however for now you'll need to birth that second particle with a Birth Flow.
You could also just keep everything in the same flow, and set the spin particle as the target of another particle, then use the Custom Properties operator to copy the spin value to the particle from its target. No need for a second flow.
You could also just keep everything in the same flow, and set the spin particle as the target of another particle, then use the Custom Properties operator to copy the spin value to the particle from its target. No need for a second flow.