This is a REALLY short post, about an amazing topic that with the assistance of Google, could easily eat up a couple days worth of reading. I just wanted to talk a little bit about the behavior of your 6500, running VS-SUP720-10G with Quad Sup VSS. The quick and dirty on VSS, for those who don't know, is it essentially stacks two independent 6500 chassis into a single logical unit. So the question that came up, when each of the two chassis have redundant supervisors... how the hell does that work?
Configuration wise, not much is different from VSS with only two sups in the mix. Aside from the fact you now have four 10G interfaces (two from each sup) in your EtherChannel for the VSL. Now the "weird" behavior comes in with what roles each sup takes, and what happens with failover. Essentially your sups break down into one of two distinctions. ICA or ICS, in-chassis active/in-chassis standby. Also note that on VS-SUP720, your ICS goes into RPR (cold standby) and you only get SSO between the two ICAs from each chassis.
Give that a moment to sink in lol. So that means if your active supervisor fails... the hot standby supervisor FROM THE OTHER CHASSIS, goes active. Alright, a little odd maybe. So if chassis A's sup fails, chassis B's sup will go active SSO. What happens to chassis A then? Or it's RPR sup for that matter? Well, bad news bears, that whole chassis reloads. It may sound bad, but if you build this right, you should have MCE (multichassis EtherChannel) built to everything off that 6500. So, theoretically the failure would be almost entirely unnoticed.