A key problem with the original punctuation of the sentence in question—
The more complex a law, the more difficult it is to comprehend, the easier it becomes for the experts to evade it.
—is that it doesn't clearly establish what connections and associations the author is trying to make. One possibility is that the author is saying that the first term and the second term, in combination, lead to the third term as a consequence. That is, A and B, working together, beget C. If we wanted to make this intention less ambiguous, we could express the sentence as follows:
The more complex a law is and the more difficult it is to comprehend, the easier it becomes for experts to evade.
Another possibility, however, is the one that Janus Bahs Jacquet points out in a note beneath the posted question. The author might be trying to say that the first term produces two results: the second term and the third term. That is, A directly begets both B and C. In that case, we could clarify the author's intent (assuming that we have already established elsewhere in the text that we would have used an Oxford comma after the second term if we had been trying to present the sentence as a true three-term series) by altering the sentence to run as follows:
The more complex a law is, the more difficult it is to comprehend and the easier it becomes for experts to evade.
There are still two other possibilities that we haven't yet considered. One is that the author means to present a sequence in which the first term has as its direct consequence the second term, and the second term has as its direct consequence the third term. To convey this idea, we might rework the sentence as follows:
The more complex a law is, the more difficult it is to comprehend; and the more difficult it is to comprehend, the easier it becomes for experts to evade.
There are shorter ways to express this third possibility, such as by inserting "and in turn" before the third term:
The more complex a law is, the more difficult it is to comprehend, and in turn the easier it becomes for experts to evade.
But repeating the second term is the clearest way to indicate that the author is talking about a sequence in which A begets B and B begets C, rather than a sequence in which A directly begets both B and C.
The fourth possibility is that the author is introducing the second term simply as a restatement of the first term, before identifying the third term as the consequence of the first term (and also of the second term, which recasts the first in other words). That is, A (or, if you prefer, B, which equals A) begets C. This is evidently the situation that the poster is interested in. The simplest way to convey this intended meaning would be to break out the second term as a visible parenthetical, either with em dashes:
The more complex a law is—[that is,] the more difficult it is to comprehend—the easier it becomes for experts to evade.
or with parentheses:
The more complex a law is ([that is,] the more difficult it is to comprehend), the easier it becomes for experts to evade.
In either instance, there is no requirement to include the bracketed words "that is" as part of the parenthetical term; but by the same token, including them will not create syntactical problems.
Or are the commas parenthetical, implying there may be laws that are complex, but not difficult to comprehend?
– Bobbi Bennett Dec 01 '16 at 15:33