This actually can be done using 2 cavity mill operations in 3X positioned corectly. when you set the cvity mill operations make sure that both operations have the 'Cut below Overhanging blank' unchecked (cutting parameters, strategy tab). for the second operation set the 'In Process workpiece' to use 3D and set a minimum material remove >0 and < depth per cut (i.e. 0.5).
I created operations and generated it on 11 and can upload the results. which NX version do you have?