I am the principal and my wife has supplementary. I can see her transactions in my M2U and so can she on her own M2U.
If you are the principal, you can go to the branch and ask them to link the supplementary to you.

edit: In M2U, under account details, look at the bottom right. Should be a link that says "View supplementary cards".

This post has been edited by -speedz-: Aug 20 2014, 06:30 PM