If we overlap symmetric islands in order to save space, how can we make two symmetric areas to have different details (like color, metalness/specularity...)?

You can't. If you want individual texture details they need unique UV layouts. I use the shared UV approach to save time (and game resources like memory) when it's not important or noticeable that they're sharing the exact same texture detail. But if I want to put in the extra work and the game can handle it, unique UVs is the "spare no expense" choice.