i think it points to one of the general principles of multicellularity: local coherence and the search-based growth processes of vascularity, neuronal endpoints, etc.

You seem to have misunderstood the concept of evolutionary pressure: there is no force somehow pushing living things to be, in your somewhat problematic phrasing, “compatible” with life, there is instead a general trend of the more-adapted-to-the-needs-of-its-moment individual to survive and reproduce over the less-adapted-to-the-needs-of-its-moment individual. Nothing about being a conjoined twin inherently augers against life or survival or in fact reproduction… in fact I’d argue that most instances of developmentally-conjoined twins survive as one pretty much bog standard surviving individual that, probably unaware, possesses some minor amount of tissue remnant from a non-surviving twin that in no way interferes with their odds of survival or reproduction. While it certainly can be a life-limiting deviation from the “normal” gestational path, it isn’t some sort of hard bar that eliminates the possibility of life or ongoing contribution to the surviving genome of the species, nor is it obvious (never mind well-established) that being a conjoined twin requires a germ-line mutation that will pass genetically to your offspring. Evolutionary pressure can’t apply in any meaningful way over a single generation.