Thanks to tonycapp, my Mac mini M1 (2020: 16GB, 1TB, macOS Big Sur 11.3 Beta 2) now has a working dual HDMI display setup. The key is using the Belkin USB-C to HDMI Adapter (F2CU038dsWHTAPL) purchased from a local Apple Store. I do not know what other USB-C to HDMI adapters might work.
Two 49-inch 4K Samsung TVs are connected, one (UN49MU6500) upstairs near the Mac mini, and a second one (UN49NU6300) downstairs with an Onkyo TX-SR393 AV receiver. The downstairs Samsung TV supports AirPlay Display, which is nice, but having a wired HDMI connection is much better.
The Belkin adapter is plugged into the Thunderbolt/USB-C port closest to the HDMI port of the Mac mini. A 6-foot HDMI cable extends from the adapter to the upstairs Samsung TV.
A 50-foot active HDMI 2.0 cable was purchased through Amazon (Toptrend, ASIN: B07KF7QDL4). This cable extends from the HDMI port on the Mac mini to the HDMI 2 (Game) input port of the Onkyo AV receiver, which in turn is connected with a 3-foot HDMI cable from its output to the HDMI 3 (ARC) port of the downstairs Samsung TV.
It took some experimentation to determine the combination of connections delivered the most stable results.
This works simultaneously on both TVs with a video signal of 2160p (3840 × 2160) YCbCr 4:4:4 24-bit (HDMI UHD color), with or without Mirror Displays enabled.
Occasionally, video glitches have occurred. For example, I have had to “reseat” the Belkin adapter, as described by tonycap. Some of the glitches are likely attributable to running the beta system software. The extent to which the hardware setup contributes to this is difficult to ascertain.
Using the Audio MIDI Setup utility app (Audio Devices), the HDMI output to the AV receiver can be configured for multichannel audio (6 ch 24-bit Integer 48.0 kHz) using the 5.1 Surround configuration. This works with Fraunhofer multichannel tests. The Apple TV app also delivers multichannel audio as PCM, or through AC3 pass through. Due to limited app support for multichannel, I tend to leave it set for 2 channels, allowing the AV receiver to apply Dolby surround effects when desired.
Thank you tonycap!