What happens if you have more than one life left and you die on the Sacrificial Slab? Also, since Sollux had two dream selves and presumably two Sacrificial Slabs, does that mean if he'd died on his planet's Quest Cocoon that he would have gone god tier?

If you die on a sacrificial slab with more than one life then your other lives die.  And yes, if any troll were to die on their Quest Cocoon with an alive dreamself then they would ascend.