Here’s a hypothetical example. Folgyr the Heavy Handed and Wevyl the Umber Mouse team up on Kurthos, the conciliatory Orc warlord. Are either of these options more legitimate, or should the DM be free to choose whichever one suits his fancy?
Folgyr: I lay into him with my mace (hack & slash). I hit with a 7 and trade blows with the Orc. I do 7 damage.
DM: … Kurthos apologizes and compliments your heritage as he connects with his axe. You take 6 damage.
Wevyl: I get behind the Orc and stab him (hack & slash). I hit with an 8. I guess I trade blows with the Orc, but… do I? He’s only got one axe, right? Does this mean he’s hitting both me and Folgyr at the same time?
DM: Yes, he’s really strong and swinging his axe all over the place as he apologizes. Just getting close to him is dangerous.
DM: No, Kurthos can only hurt one of you at a time. So you hurt him while he’s engaged with Folgyr. If you had hit with a 10, you could do the extra +1d6 damage and still not get hurt.
If Option 1 applies, is there anything about the warlord that could change to make Option 2 apply (i.e., if he only had a dagger or if Wevyl were invisible)?
Here’s another hypothetical example. Glim and Glimmer, the Orc twins, team up on Folgyr. Should one of the options below be the default response, while others only apply in special circumstances?
Folgyr: I swing my 2-hander in a broad arc in front of me. Can I hurt both of them? I hit with an 8. I do 7 damage.
DM: You can only hurt one orc at a time, so you hit Glim for 7 damage. As you pull back for another swing, Glim stabs you under your arm for 5 damage. At the same time, Glimmer gets behind you and breaks a chair over your head for 5 damage. If you had hit with a 10+, you could have chosen to hurt one of them while avoiding damage from both of them, or hurt both of them while taking damage from one of them.
DM: You can hurt both of them at once because that would be cool and your weapon is clearly designed for that sort of thing. You slash both of the twins for 7 damage. Glim stabs you under your arm for 5 damage. Glimmer staggers back, grabs a chair and throws it at you for 5 damage. If you had hit with a 10+, you could have avoided damage from both of them, or you could have done an extra 1d6 to one of them while taking damage from that one.
DM: You cut Glim on the head for 7 damage just as he stabs you under your arm for 5 damage. While that’s going on, Glimmer gets behind you and tries to break a chair over your head. Roll Defy Danger. Incidentally, if you had hit with a 10+, you could have opted to take normal damage from Glim while avoiding Glimmer entirely (no roll to Defy Danger).
Obviously, the rules do not explicitly allow some of the things I suggested in these options. I'm not shy about making house rules and trying to think about what would be fair and what would be in line with the principles of the game. On one hand, it seems like fantasy is full of examples of heroes fighting 2 or more opponents at once and having a good chance of success. On the other hand, there are many other examples where even a strong hero is in big trouble when he's outnumbered 2 to 1.