In theory, as you say, it is possible to fit 2 frame sizes. Depending on the respective geometries, you might expect one size to require greater adjustment than the other. I have 2 Cervelo bikes, a size 48 and a size 51. Placement of the handlebars requires the size 51 to have no spacers and a -10 degree stem that is 110 mm in length. Correspondingly, the size 48 Cervelo has 20 mm of spacers and a -6 degree stem that is 120 mm in length.