I have two yubikeys too, but I definitely found services which didn't allow me to register more than one when I first set them up (a couple of years ago now).
They announced a fix but it hasn't rolled out everywhere yet -- it requires converting to a new "Amazon Web Service" login that is separate from your normal amazon.com account
Oh cool, this has been my number one complaint about AWS credentials management since early 2016 or so. Over the years I've talked to a number of their project and program managers and until 2019 (that is, before the plague) they all told me that they were aware of the problem but had no plans - or even intent to fix it. Apparently the one-set-of-credentials constraint is so deeply tied into their platform that fixing it was simply deemed infeasible.
Appears that while they couldn't fix it, they have gone ahead with a workaround and made multi-device 2FA support a feature in a potential auth replacement system.
Must have been a particularly frequent and loud complaint.
