Apple Update Servers as secondary?
Is there a way to set the Apple Update Servers as a failover/fallover option to my local server (Yosemite 4.x Server.app) Update Service?
Mac OS X (10.0.x)
No, there isn't sadly.
You point your clients specifically towards one server serving Apple Updates, so would you want a failover for this service? The local server is meant to be up and running almost all the time.
Caching Server takes care of other downloads from iOS and Mac AppStore , that can be done by another server in your local network, but Software Update not.
Goodluck
Jeffrey