i am also facing the same err while appending another site for ADFS integration

Finally i found the alternate solution on that, Please modify the DefaultProviderRealm as common one don't use this in ADFS RPT

Then update the providerrealm part by using the below command

$trust.ProviderRealm.Add("Enter the site url - https://site1.domain.com" , "new URN - urn:sharepoint:site1")

$trust.ProviderRealm.Add("Enter the site url - https://site2.domain.com" , "new URN - urn:sharepoint:site2")

$trust.Update()

Then use the both new URN in ADFS RPT.

Now you can access the both site with the ADFS integration.