Does anyone else have the same issue in your organization?
I did a test in my environment but it looks like it works normally. We could add two different domain external users to the same chat group. Here is our test results for your reference:
1) search one external Teams users and chat each other.
2)Add another domain external user to our group chat
If the steps is not the same as yours or I misunderstand your requirements, please feel free to contact with us!
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