I asked my friend for advice on this question and he said that the samurai showdown was just no defense in these showdowns and all attack so I'd just give each of them the one attack from the charge and that's it, no extra reactions, no side stepping, etc.; just straight up charge and attack each other. If they survive then I would default to what I said previous.
This is assuming that they are only ever engaging in Japanese style swordplay. That would make sense for two assassins engaging in a charge together, but a guardsman with a bayonet on his lasgun is not going to fight this way. Likewise, a Cleric wielding a warhammer is not going to be practicing Iaijutsu. (or however it's spelled) What's more, both of those Samurai are choosing not to dodge of parry each other's strikes. Here's what I would do for any dual-charge situation:
- Both subjects charge their distance to meet.
- The subject with the higher Agility strikes first. They gain the +10 bonus (or +20 for BC) and execute their attack.
- If the second subject chooses to parry/dodge, they lose the momentum from the charge, and on their subsequent attack lose their charge bonus.
- If the second subject chose not to parry/dodge they retain their charge bonus and may attack as if they had charged.
- The first subject may parry/dodge as a reaction.
- Resolve wounds and critical damage simultaneously. It is possible at this point that both subjects fall.
The benefit goes to the faster (Agility) fighter because on their attack they automatically get their charge bonus. The slower subject will have to choose whether to sacrifice their momentum on a parry or risk the strike to retain their bonus.
Corvus the Moritat Assassin sees Paedrag the Cleric across the battlefield and recognizes him as his mark. Paedrag also notices Corvus, and decides to engage him to settle the matter of this impudent assassin. They charge each other to meet in a clash of steel and blood in the center of the battlefield.
Corvus' speed is remarkable, and he uses his momentum in a diving lunge to plant his Mono-Sword in Paedrag's entrails. He rolls with a +20 (having BC) and lands a hit with one degree of success. Paedrag turns his warhammer to deflect the blow, and shifts his footing. He rolls his parry, and with his reaction rolls two degrees of success. He bats the sword away, and swings his hammer low to clip the assassins shin. He rolls well on his attack despite having no bonus, and lands a powerful blow to Corvus' kneecap as Corvus made a terrible attempt to parry. Corvus is down to 0 wounds, and takes 2 critical damage to the leg. Paedrag has taken no wounds.