I have the same problem with my Raspberry Pi 4 on raspbian. And its also the Seagate Backup Plus Drive.
At first when i plugged in both at the same time, the Pi just froze. After I changed the labels, the drives dont show up when I plug them in both at the same time.
When I plug in one, it shows up. If I plug in the other one afterwards, the first one disappears and none shows up. When I boot with both connected, none shows up.
/dev/sdb1: LABEL="Data-Raid-1" UUID="770d05f9-25aa-44ab-ae02-ccc1e61705db" UUID_SUB="8ef841a3-1cca-44f5-993e-2957ab2ab137" TYPE="btrfs" PARTLABEL="PDR-1" PARTUUID="c7489545-456e-40f5-b5de-f9e1810dd35e"
/dev/sda1: LABEL="Data-Raid-2" UUID="7578b7d4-ff39-4fcd-bfda-322ecd8c9ab2" UUID_SUB="edbfa8e4-da2a-427d-971e-9a6af4d4f0af" TYPE="btrfs" PARTLABEL="PDR-2" PARTUUID="40f9f55f-2102-443f-b04a-099f2951a86a"
Any more ideas?