Why did Blake write poems in pairs or sets? Do the poems from either set critique or satirize the other set, or do they reinforce and clarify what is lacking in the other? Taken in pairs, how do the plates critique issues in Blake's London (poverty, religion, race)?

Through the use of paired poems, William Blake could emphasize an important concept by juxtaposing two aspects of that concept. He could also look at two contrasting sides of an issue, highlighting key factors in the different perspectives. Two concrete examples of his achievements are offered in Blake’s Songs of Innocence and Songs of Experience.
With the idea of “innocence,” Blake explores the purity of human childhood, but he also emphasizes the spiritual dimensions of existence. The absence of knowledge of the dangers and evils of the world make it possible to engage with the world in fresh, unique ways. This freshness enables the person to formulate a direct relationship with God, which will carry forward and shield their spirit as they gain in experience, which Blake sees as corrupting. One of the key instances in which he presents this theme is in “The Lamb.” Using the traditional symbol of Christ, Blake also considers the animal itself by addressing it directly (the literary device of apostrophe). The question, “Who made thee?” is in some ways rhetorical because in its innocence, the Lamb cannot know the answer.
Songs of Experience showcases Blake’s predominantly negative views of society, which he regards as a kind of pollution or defilement. Humans cannot exist forever in a state of innocence because their knowledge of good and evil is what enables them to make moral choices. Nevertheless, the constant struggle to do good is the unescapable burden that experience brings. A major expression of his views appears in “The Tyger.” Although Blake uses apostrophe to address the tiger and asks a similar question as in “The Lamb,” there is a reasonable expectation that the tiger could know if the same deity made it and the lamb. The tiger has the ability to take life, as a predator; Blake thus insinuates that destructive capability is a crucial aspect of knowledge.
While Blake is a severe social critic, his works are not satirical, and he is not primarily a social reformer. He does not use humorous exaggeration to make a point. More than concrete practical suggestions about how to improve society, he urges people to reconnect with their spiritual essence and live ethical lives, which may require rejecting the superficiality or hypocrisy of organized religion.