Mixing dual and single port SAS disks

Yes, the dual-port drives are fine for internal chassis use. In fact, that's pretty much all you'll find available now. HP also sends the dual-ported 6G drives as warranty replacements. They are entirely backwards-compatible.

Like ewwhite has said in the Accepted answer, it is possible to mix dual and single Port SAS disks in HP ProLiant Servers. Here's the official information from the HPE (Hewlett Packard Enterprise) web site:

http://h20564.www2.hpe.com/hpsc/doc/public/display?docId=emr_na-c01766210-1&lang=en-us&cc=us [ https://support.hpe.com/hpesc/public/docDisplay?docId=emr_na-c01766210-1 ]

HP SAS Enterprise and Midline Hard Drives - Compatibility Between Single Port (SP) and Dual Port (DP) SAS Hard Drives on HP ProLiant Servers

Issue
Is it possible to mix and match Single Port (SP) and Dual Port (DP) hard drives in HP ProLiant Servers and certain HP SAS compatible MSA storage systems, such as the Storageworks 60?
Solution
Yes. The DP drives are fully backwards compatible with the SP drives in single HP SAS domain configurations. However, only the DP drives can be used in dual HP SAS domain configurations. For additional information on dual domain SAS, refer to the Redundancy in enterprise storage networks using dual-domain SAS configurations document. Click here to go to "Redundancy in enterprise storage networks using dual-domain SAS configurations". [ https://support.hpe.com/hpesc/public/docDisplay?docId=c01451157 ]