ALSA device list: Fixed card mapping for two identical USB Bluetooth sound cards – How?

nanoPi M4 running Armbian 21.02.1 Buster with Linux 5.10.12-rockchip64. Attached are two identical B25 USB Bluetooth Transmitter Sticks which are supposed to transmit the same MP3 stream (provided by Mopidy or Shairplay Sync).

The following screenshot shows just a single B25 stick attached, the second one has yet to arrive. I expect them to all have the same USB ID ID 0a12:1004:

Here’s the problem: After each reboot the system (or ALSA?) gives the B25 stick a different card number in aplayer -l (in the screenshot above B25 is card #2, on the next reboot it could switch position and be #1).

Question therefore: How can I make two identical B25 USB soundcards to always stick to the same card number, so the assignment in asound.conf is guaranteed to still be valid after a reboot AND furthermore output to both USB soundcards in parallel?

If come across older solutions, which rely on modprobe.d, however this doesn't show any effect, so I assume it's no longer valid for Kernel 5.x. Furthermore this one.

TIA for your help!