1) Double click selection set #1, then go to Edit > Invert selection
2) Right click in drawing > Create selection set, give it a name like "temp"
3) Double click selection set #2, then go to Edit > Invert selection
4) Right click in drawing > add to selection set, select the "temp" selection set from step 2
5) Double click the "temp" selection set, Edit > invert selection
6) Right click in drawing > create selection set, call it something like "overlap"