The double hook you posted looks like it is intended for wall-mounting. If the hook is to be mounted on the shower glass, it needs to be done before the glass is tempered and installed. If you are replacing the shower glass, then look at CRL and Portals for shower glass hardware. Otherwise, find a location on the wall for mounting the hooks.

Adding -- here's an option, but more contemporary than what you posted -- Wingets Double Robe Hook in-glass