Question about unified inbox filtering

I have a question about filtering in the personal inbox. One of my users has three accounts:

- personal email account

- test1.abc (organizational email shared with him)

- test2.abc (organizational email shared with him)

When an email arrives from test1 or test2, it is labeled test1 or test2. These emails always go to his personal inbox.

He can now filter the emails using the test1 and test2 labels. But what we’d also like is for him to be able to filter out everything that doesn’t have the exact labels test1 and test2. These are, for example, conversations where he was mentioned or assigned, but which don’t have a label or either of these two labels. Is there any way to do that? Otherwise, he always has to search for them in the unified inbox among everything else.