Updated REX-Ray version in DCOS 1.10+ (v.10.0) supports a couple of shared filesystems like EFS/S3FS which support multiple mounts at the same time:
Its possible to mount same volume to multiple container on a single EC2 instance as well as use single volume across multiple EC2 instances at the same time.
Full list of REX-Ray supported storage drivers can be found here https://rexray.readthedocs.io/en/v0.10.0/user-guide/libstorage/#storage-drivers
Current REX-Ray integration in Marathon restricts the instance count to 1, this restriction is put there to support EBS volumes which can only be attached to a single instance at a time and not suitable for a shared filesystem.
Apps that use external volumes can only be scaled to a single instance because a volume can only attach to a single task at a time. This may change in a future release.
You can check a recent discussion about this in the #storage channel of dcos slack: https://dcos-community.slackarchive.io/storage/page-27/ts-1507096175000001