Right now I'm calling WriteObject(identities1, enumerateCollection:true) first and then WriteObject(identities2, enumerateCollection:true).
If identities1 and identities2 are the same type, the output looks okay. But if they are different types, the output for identities2 is blank.
What did I do wrong here?
This is quirk of how the PowerShell formatting (F&O) engine works. I've argued (apparently unsuccessfully) that this is a bug. You should find that the output is there if you use Format-List. Where I've seen this problem is that the first set of objects uses table formatting (either configured by a format ps1xml file or has <= 4 properties). Then the different object type has > 4 properties. PowerShell can't format the objects in tabular format so it just punts completely. I haven't tried this, but if you create a format ps1xml1 file for your output types, you could try to force both types to use list formatting.