One of the key questions the Eighth and Ninth Karmapas tackle in their Entrance to the Middle Way commentaries is that of the relationship between the essential natures of the two truths. In the sixth chapter of The Karmapa's Middle Way, the Karmapa asks the question, Are the two truths of the same essential nature, or are they of different essential natures?
It is important to understand that Middle Way presentations in general, and the Karmapa's stylistic emphasis in particular, are not about setting forth an assertion, or suggestion to the reader that things are or are not a certain way. Rather, the teachers of the Middle Way tradition mainly refer to other teachers of the past who have articulated the options that our conceptual minds may very likely come up with on their own when we set to grappling with these questions. Following Nāgārjuna's approach in the Fundamental Wisdom of the Middle Way, the Karmapa sets up assertions, then dismantles them with reasoning.
The purpose of this dismantling is not (to use some language of the season) to say "gotcha." It is to give our conceptual minds an opportunity to get involved in the discussion, take a side, and then be shown the inherent error of the given concept. When we finally and resolutely puncture the veils of conceptual mind's mistakenness, we see nakedly the true nature of all things. This naked, clear seeing leads to freedom from suffering.
(As a side note, I saw an interview with George Soros on Bill Moyers's PBS show recently. In a discussion about the currect crisis in the financial markets, Soros was asked by Bill Moyers, "What is the correct ideology?" Soros response was that the correct ideology is to understand that all human thought is flawed. On this particular point, I found this to be quite a Nāgārjunian observation.)
Moving along: if one posits that the two truths are of the same essential nature, the Karmapa says, there would be the absurd consequence that ordinary beings would see the true nature of reality. For ordinary beings perceive the relative truth, and they would also be perceiving the same essential nature as the ultimate truth. There would therefore be no "ordinary beings" at all, because all beings would be seers of the ultimate. (And it is seeing ultimate reality that distinguishes the noble ones from the ordinary sentient beings).
Furthermore, if the two truths were of the same essential nature, it would absurdly follow that noble ones would perceive the afflicted phenomena of the relative truth when they were resting in meditative equipoise within realization of the ultimate. For they would be perceiving the ultimate, and the ultimate would be of the same nature as the relative.
Nor can the two truths be held to possess separate natures. They are not inherently separate, because they are mutually dependent. Anything that depends on something else to achieve its own identity cannot exist on its own as an inherently separate entity.
In sum, the true nature of the two truths is beyond conceptual elaborations. From the perspective of the noble ones' seeing, both truths are merely conceptual categories.