Yes. I install spanner flanges on top of the flange all the time with no issues. The problem is that you have concrete, so you will have no way to anchor those screws to get the spanner to secure in place. I would use one of those closet flanges that has the 3 allen screws with expandable rubber ring that widens when tightening the screws. This will give the finished dimension of 3/8" above the finished floor surface, the way industry standard dictates.