I think when you analyze the relationship between the two truths, they necessarily lead to the implication that there is actually only one truth. In this way the two truths themselves are relatively conventional in contrast to the one truth which is their ultimate nature.
Anything which can be said to exist, not exist, both and neither are of the domain of conventional truth (samvrtisatya).
The fact that nothing can be found under analysis is the ultimate truth (paramarthasatya).
At face value, these two seem to contradict each other. From the side of samvrti, our judgement about the existence of things or lack thereof presupposes that such things or lack thereof can be found, which contradicts the side of paramartha, which excludes the possibility of there being anything to be found in order to make a judgement about it. This is to characterize the relationship between the two truths as a form of “symmetrical in-dependence” in the sense that they both equally are considered as distinct from each other.
In order to maintain the coherence of the two truths, we can instead consider them as complimentary. We could characterize this relationship as one in which the two truths are just different perspectives or ways of approaching the same reality. This is to characterize the relationship between the two truths as a form of “symmetrical inter-dependence,” where they are mutually dependent upon each other, such that they form constituent features of a greater whole. The problem with this, however, is to assume that there is a third kind of perspective or vantage point from which this relation of mutual dependence could be ascertained. It cannot be from within the two truths, because this would undermine their ostensible mutual relatedness by picking one side over the other. If I recall correctly, there are some Chinese Madhyamikas who posit such a third truth, but this seems illegitimate because Nagarjuna only seems to recognize the two truths or one truth with regards to the Buddha’s teachings, but never a third. This move also seems to violate the logical Law of the Excluded Middle which states that beyond a proposition and its negation, there is no other possible truth value (you might think the tetralemma breaks the law but actually I think that the third and fourth lemmas of both/neither are just permutations of the first and second).
Yet another way we can try to understand the two truths as being coherent with one another is through the relationship of “asymmetrical inter-dependence,” where the ultimate truth is inclusive of the conventional truth but the conventional truth is not inclusive of the ultimate truth. This actually accords with another way we can translate samvrtisatya: as a “concealer” truth. From within the domain of conventional truth, the nature of the ultimate is obscured or concealed, but from the domain of the ultimate the conventional exists as a member of the set of all things which cannot be found (or alternatively, are found to be empty). Although the ultimate is asymmetrically inclusive of the conventional, both are interdependent in the sense that in order to realize that all things are empty there must be things which can be found to be empty. This accords with the notion that without dependence upon conventional truth, the ultimate cannot be ascertained. Hence asymmetrically interdependent. The ultimate already includes within itself the contrast between ultimate and conventional (i.e. the two truths), whereas the conventional contrasts itself from the ultimate by obscuring it.
Hence there is really only one truth, which is the ultimate truth. And this one ultimate truth cannot be understood in the same way that ultimate truth is understood in the two truths scheme. In the two truths, the ultimate is the actual emptiness of all things. From the one ultimate truth, which is already inclusive of the ultimate of the two truths, even the emptiness of all things cannot be found, because if everything is empty then there is nothing which can be found to be empty. Hence, the ultimate of the two truths is a conceptual ultimate, a mere inference which accords with but is not identical to the one ultimate truth which is based in non-conceptual direct perception.