When an attack attack targets multiple things the controller chooses the order the damage is dealt, however allies don't die instantly the moment their health hits 0, they die the next time a player would gain priority (1.10.2b) so Metis is alive regardless of the order the damage is dealt.

There is no such thing as a primary target, they are both just attack targets. The reason this is the case is that allies don't die until state based actions are checked(1.10.2b). So in either order the ally is alive when you damage the prism.