[Quitti]

Usually items that have seperate numbers as bonuses or are wielded in seperate slots do stack though, but there are few exceptions, like wall shaker (misc item with +25siegestr) and gate cleaver (2h weapon with +50siegestr) not stacking. I don't know if this is a bug though. That combo gives only 50 bonus siege strength, so you need some random commander to hold them seperately.