Two lines, two times of day, and a sharp change between them. Morning is "like a gold chain": light, delicate, something you might drape over your fingers. The image suggests a thin, glinting beginning, maybe sunlight catching on things, maybe a sense of value and calm. A chain is also linked and continuous, so the morning feels like it unspools slowly, one bright piece after another.
Then noon arrives "like a roundhouse," which is a big swinging punch. (A roundhouse can also mean a building where trains are turned around, but the punch fits the sudden jolt best.) The gentle shimmer gives way to something heavy and blunt. The sun at midday hits hard, and the day stops being pretty and starts being forceful.
The speaker doesn't say how they feel, but the contrast does the work. There's admiration for the morning and a kind of wary respect, even dread, for noon. The title, "One, two," sounds like a boxer's combination, so the poem itself lands like a quick jab and a hard follow-up.