How can I configure 2 ThunderBolt links for a Virtual Bridge (Sequoia 15.3.1)?

I was able to solve the original problem of getting both Thunderbolt links (of the ThunderBolt Brigde) to show as Active. However, once I did this the Kernel_Task on the Mac mini (2018) started running at about 400% CPU. Apparently, this is a preventative measure to stop the CPU getting too hot (see If kernel_task is using a large percentage of your Mac CPU - Apple Support (CA)). In any case, the SMB server status still didn't show that multichannel was active.

It's possible that multichannel SMB is not possible between Macintosh computers.

- Pie Lover

An alternative to using multichannel SMB is to use the built-in macOS Archive Utility to compress the source directory prior to any transfer. This is particularly useful for backing up a Photos Library since the library is actually a "package" containing a large number of small files. Archive converts the package to a single file which is more amenable to efficient transmission.

The Archive file can itself serve as the backup or, if desired, the original file can be recovered by double clicking on the Archive file once received.

- Pie Lover