Sorry for the delay… But thanks for asking again. I now tested it (but it may be not representative for all Android devices - as written before, I suspect my phone doesn’t have a “secure enclave” and therefore stores passkeys only in my Google account!)
So, for the Bitwarden web vault, I could create a 2FA-“passkey” with my phone. Okay, it’s a “non-discoverable” credential, and I really don’t see it anywhere (I suspect, it’s “stored on” or rather associated with my phone/device itself).
Then, I could also create a “login-with-passkeys”-passkey with my phone. As just written above, this, as a real passkey, I can only store in my Google account (Google password manager handles this on Android phones - but I don’t get offered any possibility to store it on my device directly - restriction of my phone, I guess).
And now, I can login using both types of credentials. So they don’t get overwritten.
I think I just answered this. – Would be interesting to know, though, how it would be handled by an Android device, where you can store the “login-with-passkeys”-passkey on the phone (device) itself.