Good question Boothy.
The crux of your question is dealt with in the interpenetration while moving rule allows you to pivot through an enemy unit when moving (charging) but not move through it. So any unit that overlaps an enemy unit while charging would be deemed to have contacted and thus charged the enemy unit it just overlapped.
But as you suggest the typical Regiments of 100mm (wide) x 80mm (deep), 120mm (w) x 80mm (d), 125mm (w) x 100mm (d) and 150mm (w) x 200mm (d) can indeed do what you suggest. But Troop sized units either end up with their flanks flush with the unit that just charged them (i.e. are in contact with the enemy unit or they overlap them - the 120mm wide and 125mm wide troops are the ones that overlap, while the 150mm wide Chariot Troop ends up flank flush and in contact as well). Thus Regiments can do what you suggest but Troops can't do it. The reason is all to do with the relative position of the axis of rotation of the Regiments (which are twice the depth of a Troop) being a lot further back that the axis of rotation of a Troop. So the deeper your unit the easier it is to do what you suggest; the Chariot Regiment (150mm wide and 200mm deep) is the most efficient at this as despite the fact that it starts only 1" away from the unit that just charged it once it rotates 90 degrees it ends up 2" away from that unit (the centre of rotation being a massive 5" back from the unit that just charged it.
So, it is possible in many cases provided you can see the unit off to the side you want to charge. If by some chance that unit is located just a little bit further forward than the unit that just charged you then you will easily hit the adjacent target unit in the flank. Booyeah!
In V2 of the rules the removal of 1 pivot from your unit that was just charged will make this very difficult, unless
you have a height advantage over the unit that just charged you, and if the adjacent enemy units are front-edge flush that there is a gap wide enough between the adjacent units that allows you to fit along the flank. Hard to explain in words so I hope they put lots
of diagrams in the rules that explain what is and what is not legal. Personally, I'd rather the rules just said that if your unit was charged it can only charge the unit that just charged it (and if it was not charged in the previous turn that it could charge anything in range that it could see).
So after my somewhat clumsy delivery of that wonderfully mind bending paper (digital) tiger lets alleviate the mental anguish with some lovely pictures of my latest additions to my Kingdoms of Man army. This week saw completion of 80 pikemen (two Pike Phalanx Hordes!!) which I cranked out in just over 4 days. They are Perry Miniatures European Mercenaries painted in Swiss livery. Next up some more
infantry. Best wishes for an action-packed weekend of quality gaming/painting.
Pike Phalanx Horde in Lucerne colours
Pike Phalanx Horde in Uri colours