Now that you point it out, it’s just as ungrammatical as Vergil’s urbem quam statui vestra est (Aen.1.573). The qui does not introduce a conventional relative clause, since simul is clearly functioning not as an adverb but as a conjunction (“as soon as”), so that the qui, while obviously providing the subject of stravere, is then left stranded. I don’t think I’ve ever noticed that before, though I certainly should have.
I suppose we could say that the qui is effectively brought inside the simul clause (despite the fact that its antecedent is divis), and is lost sight of once that is over. It’s an interesting anomaly. I rarely look at Latin grammars these days, so I can’t say if they discuss it.
Whatever was I thinking? It’s perfectly simple, it’s basically no different from quae cum ita sint (“[and] since this is so”), and there’s nothing anomalous about it at all. I don’t know why I said the qui is left stranded. It’s done its job, and there’s no residue. It switches the subject to the gods, that’s all.