There's a good read on the Mesa website about how they "invented" the whole Mark series cascading preamp. You might have to look for it, because it might be within the "mesa story" or whatever. But you're basically on the right track. It refers to having one thing overdrive another thing. It's kind of like doubling or tripling down on the overdrive. Like if the first stage is capable of a 10x boost then the second stage is really a 100x boost. If you want all the real tech nerd descriptions of it, check out some amp/pedal forums.