DRAG DROP - Drag and drop the Cisco WSA methods from the left onto the correct deployment modes on the right. Select and Place: 

You could use an explicit setting (browser config/PAC/WPAD) or transparent (WCCP/PBR) to point to the first proxy and then the first proxy will route based on category to one of the two upstream proxies based on your policy configuration. Reference: https://community.cisco.com/t5/web-security/route-to-wsa-based-on-destination/td-p/2491179