You have a couple of options here:
Option 1: You can create secondary zones. On the server that hosts x.local you would create a secondary zone for y.local then repeat the other way round. if you do this then you will have to allow Zone transfer from one zone to the other from the zone transfer tab
Option 2: you can create a stub zone on each. On the server that hosts x.local you would create a stub zone for y.local then repeat the other way round adding a stub zone for x.local on the server that hosts y.local
Option 3: you can create conditional forwarders on each. so on the server that hosts x.local you can create a conditional forwarder for y.local then repeat for the server that holds y.local
https://technet.microsoft.com/en-us/library/hh831667(v=ws.11).aspx