Due to the Duo Hub utilizing two USB-C ports on your MacBook, you can extend to two additional displays.
If you have 2 x HDMI monitors, the HDMI monitor is connected from the HDMI port on the hub, and the second HDMI monitor is connected from the first primary USB-C (Thunderbolt 3 pass-through) port, with a USB-C to HDMI cable. (See the image below)
USB-C to HDMI cable.
If you have a DVI monitor as the second monitor, please get a USBC to DVI cable, such as below:
If you have a MiniDP or VGA monitor as the second monitor, please get a USBC to MiniDP or VGA cable, such as below:
As long as one monitor is connected via HDMI-to-HDMI and the other is connected via USB-C to the display monitor, you will achieve two displays through the Duo.