The short answer is no. And even if ENVI could separate the different "objects" in a multi-polygon ROI, the ROIs that are generated from the band threshold tool are pixel ROIs. So, ENVI doesn't recognize any relationship between "objects", as you describe them, within that ROI. It just sees a bunch of pixel addresses. Y ou can tell this by looking at the ROI in the ROI Tool. Under the Pixels column, you'll see some number listed, but you'll see 0/0 listed for Polygons and Polylines.