Well, the unit falling back will be subject to "Trapped!", as described on page 48 of the rulebook, unless there is a way for it to move through the unit behind it. Remember that models may not move through the space occupied by another model, so there either must be sufficient space between the models in the other squad, or there must be a path around it within the fallback corridor. If that unit that is falling back can successfully move passed the other unit, all is well.
This is a good reason to leave at least 1" between all of your models. To move through a friendly unit their bases must have room to pass. You can have two units intermingled as long as all units in each unit retain coherency. It also helps to protect from blast markers.
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