The real crime here is that there’s no way to share a Photo Library between two people, or sign into a different Apple ID for iCloud Photo Library.
Yes, it is.
I’ve spent years trying to identify if there’s any technical or logical restriction that would make library sharing infeasible.
Keep in mind that before iCloud, we could sync all the phones we wanted to the same Mac, and accomplish the same goal, albeit much less efficiently.
And I’m not trying to share with a random person, but rather someone who is already part of Family Sharing. So we’re all writing to the same iCloud Drive, for example.
And of course it’s not like a single library can’t already be fed by multiple devices.
So it seems purely a matter of permissions or identity, if anything at all.
I keep hoping with every release that this will be a big new announcement And I don’t know why there aren’t more people clamoring for it.