Let's imagine this situation : my Roman lancers and Armoured Cavalry vs one single Roman lancers.
1) Step 1 :
My Lancers charge. The enemy Lancers are meant to charge too. This is a balanced charge. Ok.

2) Step 2 :
The enemy lancers are in melee. My Armoured Cavalry charge. There is no flank charge as the game rules define it in this case, but however, shouldn't my Armoured cavalry 'always' win the Impact or at least get some extra Impact PoAs ?
The first ranks of the enemy are in melee, fighting. The last ones are immobile, behind, unsteady, a bit disordered, not seeing well what is happening in front and refilling the first ranks to replace the dead cavalrymen. And an enemy Armoured Cavalry crashes into it (I would say into his side, if not into his flank). This time the last ranks can't charge (ie move forward) and have to absorb the shock (with or without lances).
Even if everything during a given turn is meant to happen at the same time or in another order, it might seem odd that a single unit can manage the impacts of two enemies so easily during one turn.

What do I miss out ? That the last ranks of the enemy lancers can actually charge the second unit ?
How should I understand this situation ?
(edit)
PS : The same thing happens in P&S when, for example, two Determined Horses are already in melee and a Horse charges.


