@Seat 32F
Also, the normal rules for safety for dealing with engine failures are designed around the assumption that most engine failure scenarios are independent - designers go to great efforts to make the two (or more) engines on a multiengine plane as independent from each other as they can. And operational and maintenance procedures exist to try to eliminate any remaining "common causes" (things like dont change two at the same time, in case the same maintenance error affects both, and so on).
Now, if a plane fly through a dangerous level of ash, that condition applies to all engines, and all may fail (or be affected) equally. So the aircraft may not have the option for a flight at a lower altitude; it may become a rather large, and not very efficient, glider. Which is not a good outcome, from a continuation of flight perspective.