Would it be possible to ascend twice if you were asleep on your quest bed, put your dreamself on their quest bed, and had both die at the same time?

I don’t think it’s possible to ascend twice.  I also don’t think that their are multiple ascending stones.  Since Skaia already knows everything about the session I think that Quest Beds are give to those that will need them and Sacrificial Slabs are given to those that will use them, and no one gets both. But that’s just my headcanon, there’s nothing mentioned about it in canon