Aaaaaaaa.... OK Yes I see!
But most be some easier way to break this in parts again?
Not by keeping the cross brace intact, but you have a few options, easiest - delete part and mirror copy the good side, Or - use the facet edit tool to select the cross brace, hit delete key, space bar to get back in normal selector arrow, select part and explode once, selection palette should now report two ACIS objects, Or ................ way too much hassle to try recover the cross brace as well as the two beams through intersection and facet editing.
You would have needed part tree to be active in order to recover from the addition (i.e. prevent the addition taking place).