You can certainly group arrays together for managment purposes. You DO NOT have to merge the storage pools, however (striping of volumes across multiple arrays).
That being said, you cannot perform intra-group replication with Nimble Storage arrays today. If you would like to replicate, both arrays need to be in separate groups.
Information on tagging data subnets can be found in the administration guide under the resources area in Infosight for your appropriate NOS version.