I just stumbled across this and figured I’d share. The release notes for ASA 9.1 (x) was updated on March 31st which prompted me to browse the document. As it notes, the ASA 5512-X, ASA 5515-X, ASA 5525-X, ASA 5545-X, and ASA 5555-X now support 2-unit clusters. Clustering for 2 units is enabled by default in the base license but for the ASA 5512-X you need the Security Plus license. This is a feature that I know many people were waiting for!
Here is a link to the document: http://www.cisco.com/c/en/us/td/docs/security/asa/asa91/release/notes/asarn91.html#pgfId-586890