We do this internally at my company with Qt5.15.2. We built Qt from sources for both Apple Silicon and Intel first and used those outputs in our app builds. The tl;dr is that you have to build your Apple Silicon binary with Qt for Apple Silicon, build the x64 binary with Qt for Intel, and then use the Apple "lipo" tool to stitch the two together. This can all be accomplished on an Apple Silicon Mac.

The tricky part for me was figuring out that I had to switch the shell to x86_64 to run the compiler tools for the Intel build, after running the compiler for the Apple Silicon build under an arm64 shell.