I think the short answer is NO.
When a Syno has an eSATA drive connected, the default behaviour is to try and mount the drive in a prescribed location. If you tried to connect two together they would both try and manage each other's volumes (if they saw anything down that channel at all).
The issue really is the shared management between two controlling units. If you really are hoping to replicate terabytes of data copying/replicating between two units is not going to be fast ! (no matter how you connect the units eSATA/iSCSI/NFS).
Depending on the type of data you have (dynamic or static) one solution for mainly static information is to use Rsync between the two units and periodically run a backup this will work well and I do it between my RS810+ and DS209+. But it's no "Failover" solution.
* Model: RS-810+ * Firmware: 4.1-2567
* Model: DS-209+ * Firmware: 4.1-2567
* Modification(s) DS-209+ - Upgraded RAM 1024 MB.(+Optware +OpenVPN 2.1)
* Drives Seagate ST31500341AS + ST2000DL003.
* Network: 1000xBASE T, Full duplex. [Bonded on 810+]
* Services enabled: Most of them!