Bleed Them Dry.

One set of runes is white and is etched directly onto the armor while the other set is blackish purple and floats above the scales. Their wide jaws that split their face shared an identical grin that displayed their numerous black teeth. 

If there is any physical difference between them, it is that the white halo of light atop their heads is not identical. One halo has one black sphere attached to it while the other halo has two black spheres. 

The other difference is within them. If one can see within them, you will notice that even though they both have inner worlds, their inner worlds are different. 

One inner world is ringed with a red crown and contains nothing visible. The other inner world has a landmass in it. On that landmass is a green elf, a battle sage monkey, a multicolored spiritual entity, and an elaborate construction. Beyond these differences, the two of them might as well be the same entities. Even their soul fluctuations are identical.