  This is not the epistemological question of how we could  know that these are the pairings (although this is  troublesome, too). The question, rather, is metaphysical: in virtue  of what are these the pairings? If minds were, like bodies,  located in space, causal pairing could be achieved by the relative  spatial locations of the substances (Bailey et al. 2011). Particular minds might be inside or “inhabit” particular bodies. But if minds are non-spatial souls,  relative spatial location is unavailable to fill the pairing role.  And since M1 and M2 are, by hypothesis,  exactly similar, we cannot appeal to the different intrinsic  properties that they might possess.