"""Which one should I use as the tomo nagura for slurry and which one should I use as the base stone? The one on the left is 1/4 to 1/3 inch thick while the one on the right is 1/2 to 3/4 inch thick. """
In general your tomo should be slightly softer than the base stone, just rub yours together and determine where the slurry is coming from. The quality of the slurry from whatever stone is partly dependent upon the inherent quality of the stones being rubbed. This is apparent if you used a plain old sandstone as a tomo from the hill behind your house on your cherished Jnat or Arkansas. For us looking at photos of the two Jnats, who knows which one is finest or if either is a razor grade stone. Rub them up and tell us which is which. If both stones are from the Ozuku mine, and they are thin like that (suggesting modern digging), they were each sold as base stones. I have one that looks like a brother to the one on the left, mine is really really hard. It is silly to rub two super hard stones together, all they do is scratch each other like cats fighting. No one really wins. Usually the tomo provides the slurry and thereby helps to keep the base stone intact and flat while providing the slurry.
Alex
I rubbed both stones together using the left one as the tomo and the slurry matched its color (of the one on the left) so I'm sure the one on the right is harder then the one on the left. I'll shave with it's first edge tomorrow. The one on the left produces some great shaving edges including one that I considered my best yet. And the razor (atleast one one side of the razor moreso than the other) seemed to have this suction or sticking the further I got along with honing.