Question about Jack Stand Weight Ratings

When you see the ANSI weight rating, is that the weight one stand can support by itself, or is that the weight both of them can support together? In other words, could two six-ton jack stands used together support up to 12 tons, assuming the weight was evenly distributed?

should be rated per stand.

I sell them. The rating is for each stand

Jack stands are rated per pair, as defined by the ANSI standard for Portable Automotive Lifting Devices. If you need verification, check the standard or contact the committee.
http://www.normas.com/ASME/pages/PALD.html
http://cstools.asme.org/csconnect/CommitteePages.cfm?Committee=L01800000