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I’m not sure without trying it, but if it is going to work, the syntax should be
&unobserved_by_user_id=hydrophilus,lemonsqueeze
Just a single comma-separated list instead of two parameter instances.
If that doesn’t work, try using the user id numbers instead of user names.
I’m actually not finding this parameter described anywhere in either part of this wiki. So if it does work, we need to document it in the appropriate category I guess.