I made a 5 piece of twice born star armor. While it does let you have 2 mundus stones active at once, whatever one you had working before you don the armor seems unchangable after you put it on.
Example: You have antronoch active. You put on TBS. You pick up thief, you now have both (as it should be). But, if you try to get say shadow, it will always overwrite thief and antronoch will never go away. In other words you can only change mundus #2, #1 seem immutable. Is it supposed to work like this?