Multiple Apple id do not appear to be possible

Apple support notes suggest multiple user accounts on an iMac can use separate Id. Having tried to download through iTunes my wife's music which is on the cloud I get a message " This computer is already associated with an Apple id". It goes on to say. No downloads will be allowed for 90 days with an alternate id. It is not clear whether that is in this account or on the Mac as a whole.

If this is the case how is iCloud managed in two accounts as it also uses the Apple id login.

I would appreciate any help someone could give me.

Thanks in advance.