Update y is a cumulative update the supersedes update x - therefore, you only need to install update y.
https://www.ajtek.ca/wsus/how-to-setup-manage-and-maintain-wsus-part-8-wsus-server-maintenance/
Also, a little excerpt from the documentation of WAM:
There has been a ‘best practice’ for years where declining superseded updates should only be done after
the superseding update is approved and the superseded update has been installed on all systems. We at
AJ Tek are looking to change that method of thinking into a blanket declining of superseded updates. If
you think about it, there is no need to keep the superseded updates for that long. Once testing of the
superseding update is complete, and you are choosing to approve the superseding update, there is no
reason for the clients to continue installing the superseded update – they should go directly to the
superseding update which is now approved.