You can create super metrics to total the read/write IOPs for all the vCenters. Here's an example for the Read IOPs, assuming you are using the Datastore|Read IOPS metric:
sum(${adaptertype=VMWARE, objecttype=VMwareAdapter Instance, metric=datastore|numberReadAveraged_average, depth=1})
Total Write IOPs:
sum(${adaptertype=VMWARE, objecttype=VMwareAdapter Instance, metric=datastore|numberWriteAveraged_average, depth=1})
Assign the super metrics to the vSphere World and enable them in the policy.