Think that is a potentially smokey typo....
Try:
The ground wire would go to the + of the -24Vdc supply and the - of the +24Vdc supply.
What should happen is this:
The -V connection on the scan amp goes to the 24V- terminal on one of the supplies, the 24V+ terminal of that supply goes to the OV terminal on the scan amp AND to the 24V- terminal on the second supply, the +24V terminal on the second supply goes to V+ terminal on the scan amp.
The inputs on the supplies are wired in parallel.
Regards, Dan.