You mark the half way point on the blade, so that you can only hone the top half of the razor with the 12k. As you hone, just keep the marker line, on the edge of the stone as a guide. Then compare the stria on both halves. If both stones are in fact 12k or the same grit, the stria should look the identical.
If the stria on one half is finer than the other, then they are not the same grit, and you will know now know, which one should be used first in progression.