Can't seem to find this noted anywhere. Is BTRFS capable of being accessed over iSCSI by two hosts at once? (Think how esxi hosts access same datastores)
Access by multiple initiators requires a clustered file system. The btrfs develoeprs do not support this and do not want to support this. From the FAQ:
No. Btrfs's main goal right now is to be the best non-cluster file system.
If you want a cluster file system, there are many production choices that can be found in the Distributed file systems section on Wikipedia. Keep in mind that each file system has their own benefits or limitations, so find the best fit for your environment.
The closest cluster file system that uses Btrfs as its underlying file system is Ceph