Units that are Commanded onto a hex containing both a Garrison and a Legion (or Skeleton and Horde) must resolve both combats sequentially; they cannot retreat or move away between combats.

This is supported by the following points:

• From page 21:
  "If your Units enter a hex with any enemy Units, combat begins. If there is an Imperial Garrison or Chaos Skeletons, fight those before fighting any Legion or Horde; if there is more than one Horde, or more than one Legion, fight them in their initiative order from lowest to highest.
  Each combat counts as a distinct and separate combat, so you can gain Before Combat and After Combat effects multiple times."

• There is no mention in the provided context of a retreat or movement option between sequential combats when multiple enemy types are present in a hex. The rules specify that you must fight the Garrison/Skeletons first, then the Legion/Horde, in order.

• From page 36:
  "Repeat Clash until one side is destroyed. ... After all destroyed Units have been placed in the Graveyards, if there are only enemy Units on the hex, they remove any Haven and Defenses (Walls & Towers) there immediately."

Summary

Units must resolve both combats in sequence when entering a hex with multiple enemy types (Garrison and Legion, or Skeleton and Horde). There is no rule allowing retreat or movement between these combats; both must be resolved before any further movement or actions.

Source: pages 21, 36.

Source: Core Rulebook (pp. 21, 32, 34, 36, 57)