Each individual stand has rating on it then when added up equals the capacity of the pair. They're not measured as a pair. A set of 4 - 3 ton jack stands would be 4x3= 12 tons of capacity. A pair of 6 ton jack stands does not mean two three ton jack stands, it means two 6 ton jack stands.
For example, I have a pair I picked up from autozone a long time ago. They're sold as 6 ton jack stands and the capacity is listed as 6 tons per stand, aka each stand can support 6 tons:
http://www.autozone.com/featured-it...ir-6-ton-jack-stand/234799_0_0/?checkfit=true
I don't think that's as standard as you're getting at.
Although a jack stand rated for 3 tons technically should hold a lot more than that weight on it's own before failure, many jack stands are rated based on 2 being used on a single axle.
See this HarborFreight pdf catalog. Notice on the first page the small print... "evenly distributed across 2 jack stands"
http://manuals.harborfreight.com/manuals/38000-38999/38847.pdf
and the one that I posted from sears is the same:
https://redirect.***.com/?format=go&jsonp=vglnk_147673413805518&key=7594a612deafba047f2964b289a9aebe&libId=iueh1xs30100zctg000DA2vsclbko&loc=https://forum.ih8mud.com/threads/onsc-chat.817372/page-132#post-10627361&v=1&out=http://www.sears.com/craftsman-professional-4-ton-jack-stands-one/p-00950163000P?plpSellerId=Sears&prdNo=1&blockNo=1&blockType=G1&ref=https://forum.ih8mud.com/threads/daily-funny-pics.364133/page-318&title=ONSC Chat | Page 132 | IH8MUD Forum&txt=Sears.com&loAsUuid=iueh1y65-2cb1edbc-edf4-42f6-a373-0a5fa18b9d60
(8000 lbs COMBINED) capacity (listed as 4 ton jacks).....