Corp-X and Corp-Y belong to the same side.
Corp-X starts in locale A then moves by a local road through locale B into locale C.
Can Corp-Y now move into locale A?
The rules state that 'after the corps enters a locale (even if just to pass through it), no other units can move into or through the reserve area of the locale for the rest of the turn'.
That would seem to indicate that Corp-Y couldn't move into locale B or C but seeing as Corp-X moved out of locale A and didn't enter local A (and then pass through) that Corp-Y can enter locale A.
So long as Corps-X is a corps of 2+ units moving by road, that restriction applies.