Hey @Armenito 
You are correct – In order to view The Connect Boards Column and information within The Mirror Column, you need to have permission to see all of the boards they are connected to.
This means that if you do not want them to have permission to view Board 1, then they will not be able to mirror information from Board 1 into Board 2.
However, a potential solution in that case would be to reverse which board these columns “live” in! Basically, have the columns that you want the people to be able to see built directly into Board 2 and then mirror these columns into Board 1 instead.
If you’d like, you can always share your feedback about this mirroring capability to the Feedback Section of the Community so our Product Team can take it into consideration as they make future changes to the platform!
If you would like to submit your own feature requests or feedback, you can always add a post or vote and comment on one that already exists here: Feature Feedback and Requests
I hope this helps! Remember to mark as solved if this answers your question 
Cheers,
Jenna