A week! Damn, that would persuade even me to read the manual! ;) As Rochr says, select both objects, after you've made one positive and the other negative. Click the Group icon, or choose object/group, and voila! Note that if one of the objects is already part of a group it don't work. The mat of the positive object is, obviously, the outside mat, and the mat of the negative object becomes the mat of the hole.