If only one monitor is working through the Type-C Dual HDMI Adapter, the most likely reason is your Mac's own display limit rather than the adapter itself. Base M1 and M2 chips cannot extend the desktop across two external displays through their Thunderbolt 3 ports, so the Type-C Dual HDMI Adapter is limited to mirrored displays or a single external display on those Macs.

How to fix dual display problems with the Type-C Dual HDMI Adapter

1. Plug the Type-C Dual HDMI Adapter into both USB-C ports on your device. The LED indicator illuminates when the adapter is connected.
2. Confirm your computer is supported. The dual USB-C connector design is measured specifically for MacBook laptops with Thunderbolt 3 ports and for the Mac mini with four Thunderbolt 3 ports. It is not designed for Windows computers.
3. Check your Mac's chip. Click the Apple menu and choose About This Mac. Dual extended displays work on M1 Pro, M1 Max, M2 Pro, and M2 Max models, up to two extended displays at 4K and 60Hz. Base M1 and M2 Macs are limited to mirrored displays or one external display, which is a limit of the Thunderbolt 3 and USB4 ports on those Macs, not of the adapter.
4. Insert two HDMI cables into the adapter's HDMI ports. An HDMI 2.0 cable is recommended for the best results.
5. Use direct HDMI to HDMI connections only. The Type-C Dual HDMI Adapter does not support DVI or VGA.
6. Do not use an HDMI splitter or daisy chain displays together. Splitters and extra adapters are not intended for use with this adapter because of power requirements and image quality.
7. Switch each monitor to the HDMI input you actually plugged into.
8. Open System Settings and go to Displays to detect the second display and set it to Extend rather than Mirror if your Mac supports extended mode.
9. Try a different HDMI cable on the display that is not working, or swap the two cables to see whether the problem follows the cable or stays with the port.
10. Connect your charger to the pass-through port to keep your Mac powered, up to 60W max. That side USB-C port is for pass-through charging only, so it does not carry data or video.
11. Use the included silicone cover when you run two 4K 60Hz monitors with power connected. It is normal for the Type-C Dual HDMI Adapter to get hot, and its standard temperature range should not exceed 52 degrees Celsius (126 degrees Fahrenheit). Removing the cover does not affect performance.
12. Expect some slowdown at the top settings. Depending on your computer's specifications, streaming at 4K and 60Hz for extended periods can cause slight lag in video output, especially with video running on both displays at once.