yes, it is required to install the OS from the ovirt-node-ng iso on the first disk and then select the second empty disk for gluster bricks/volumes during the wizard phase.

Please note also that when you run anaconda from the iso it by default selects both the disks as target of the OS install (with a warning), so that you have to click on the second disk (and deselect it this way) and proceed with the install on only the first disk