I have a customer who has 3 servers with 2 FC HBAs each. He wants me to connect a Dell ML6020 with 6 LTO-4 FC tape drives so that each server is connected to two of the drives.
I've done library sharing many times, but always connected through a switch so that all servers see all tape drives and the library. Incidentally, the Dell (ADIC) unit uses one drive to talk to the library (or more if you partition the library into multiple virtual partitions).
I have never used Netback and don't need to know much about it - just if it will support this configuration and if so, the basics of how it can work like this.