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As the other posters have already described, 6500s can be dual "stacked", although as they also described, it's called VSS on the 6500s.
Unlike 3750 stacking, which uses special copper cables, 6500 VSS is done using 10 gig ports which can use the various 10 gig transceivers for those ports (copper or gig). Not all 6500 10 gig ports can be used. Those on the VS-S720 can be (one of the two 6500 Sups that supports VSS; the other is the Sup2T) as can those on the 6708. If I remember correctly, 10 gig ports on the 6704 can NOT be used. Don't know about the other 10 gig cards.
I believe VSS can interconnect 6500s at ZX optic distances, but as normal usage is to dual home every other connection to each chassis, having the 6500s other than near each other would make it difficult to dual home all the other connections.
VSS also requires one of the recent vintage IOSs too. Also note, a "normal" 6500 has a single Sup in each chassis, but 6500 VSS now also supports dual Sups in both chassis.
Normally, only two 10 gig links are used to interconnect 6500 VSS, this to avoid a single point of failure using just one link. Assuming everything thing else is connected to both chassis, only control traffic normally passes across the VSS connecting links. (VSS tries to keep data flow egress on the same chassis it ingressed on.)
6500s in VSS configuration add a unit prefix of 1 or 2 to all the interfaces; the 1 or 2 being the physical chassis. Logically, the VSS configuration functions as one device and is "seen" as one device by other connecting devices (much the same as stacked 3750s.)
