I understand MM I maximizes firm value and MM II minimizes WACC, so essentially they are both the same thing. Am I missing sth here?
They do not really say the same thing. The first is in reference to Mkt value of a co. - you cannot create value by changing the capital structure. The second is in reference to cost of capital (WACC) - cost of equity has a linear relationship to the D/E ratio (ie capital structure) - There is NO change ind cost of capital by changing the capital structure. Just remember MM1 is in reference to Mkt value and MM2 is in reference to cost of capital. Neither change as a result of changing cap structure.
^ without taxes this is true, but factoring in taxes M&M Props I and II do show changing the cap structure creates value and lowers the cost of capital which then moves you on to the static tradeoff theory that factors in costs of financial distress and a theoretical optimal capital structure