I own two OnlyKeys, but it looks like their serial number is the same, and I cannot tell if that is by design or because I have restored one to the other.
Problem: when I have both keys inserted at once, an application (like KeePassXC) sees them as one because they report the same serial number. If I insert only one key at a time, it shows the same serial number for each. I therefore cannot choose which device to use for challenge-response, since KeePassXC is essentially choosing one device at random to be the one it talks to.
If I insert one OnlyKey and one Yubikey, KeePassXC sees and lists both, allowing me to choose one or the other.
Background: intending to use the second as a backup, I did a full backup/restore of the first to the second. It worked. I’ve decided to use the second one for something else now, and did a full firmware overwrite in order to start from scratch. (Using 2.1.1 on both.)