I'm not sure the dream self thing is accurate (if the dream self were on the sacrificial slab). I don't recall Waking Self wounds ever taking effect on a Dream Self. If Dream Selves were injured and died in sync with their respective players, then what would be the point of having one?

After Vriska was beat up by Aradia her dreamself started to bleed too. And they don’t die in sync.  There’s a bit of a delay to give other players time to kiss the dead one to wake their dreamself or to get them to the questbed/cocoon.  Obviously if they died at the same time they wouldn’t be able to ascend.