If you want to reference one (master) Nexus repository from another (slave) Nexus repository this is what you have to do:
Add 2 proxy repositories to the slave Nexus, one configured for releases and one configured for snapshots. Set the remote url in both to:
Then add the releases proxy to the public group and the snapshots repository to the snapshots group.
Thank you to Damian Bradicich of the nexus-users group for these instructions.