Many DAS devices include the ability to daisy chain additional units together. Often in the specifications you will see these listed as miniSAS (external) for expansion. However, the problem comes with throughput on your actually RAID controller. If you've got one DAS connected with a single SAS cable, and that first DAS is already running lets say 16 hard drives, that's a lot of drives to be pushing through that single SAS connection. Then you go and add another 16 drive DAS through that same SAS connection and your throughput is going to be really suffering. It may be better to find a RAID controller with two external miniSAS and set up two separate DAS instead of daisy chaining them together. I've also seen people utilizing two individual RAID controllers with miniSAS external connections for running multiple connected DAS devices and arrays.