The terms of comparison may each be a single entity, or one or both may be whole configurations. “Single” here does not mean “spelled as one word”; it means a unit not analysed as a structured arrangement of components.
Single against single: “The child’s face is like a flower.”
Compound against compound: the scene of swords flashing through dust compared with stars falling through darkness.
A single subject may also be compared with a compound image. Analysis begins by locating the unity of each side.