Sarah's poem starts with a piece of common wisdom: being alone and being lonely aren't the same thing. Alone is a fact about where you are, with nobody else in the room. Lonely is a feeling, the ache of wanting company you don't have. Most people say this to comfort us, as if you can enjoy one without suffering the other.
Then the poem turns. The speaker points out that "they" never mention what happens when you're both at once. The poem doesn't answer that question, and the silence is what makes it hit. The speaker seems to be living the answer, and it's too heavy or too familiar to spell out.
The title, "deadly duo," makes the pairing sound like a team that teams up against you. The mood is quiet, bleak and a little bitter. The lowercase style and the plain, chatty voice make it feel like a private confession, something said honestly rather than dressed up.