2 display monitors on 1 HDMI output.

You need to connect one monitor to the HDMI port on the MacMini and you need to connect the other display to one of the USB-C ports with a USB-C to HDMI adapter.

The only adapters that may be out there to connect two displays to a single HDMI port will only mirror the displays.

+1 woodweister50 is spot on.

The Apple adapter is one example > USB-C Digital AV Multiport Adapter - Apple

Plus there are many aftermarket USB-C to HDMI adapters will also work.

for example > https://www.target.com/p/anker-powerexpand-usb-c-to-hdmi-adapter/-/A-79804194?

and > https://www.bestbuy.com/site/insignia-usb-c-to-hdmi-multiport-adapter-white/6424884.p?