i'd have to disagree.
You get attacked by one creature. You Armor deflects some damage...you knock some away with your sheild / sword, etc.
You get attacked by the other...you armor deflects nothing...and obviously you're busy fighting the other to block with your shield / sword, etc.
Why is your armor useless against the second monster? Suddenly your thick plate of armor is like a peice of paper because you weren't looking? I think not.
The fights should go something like this...
DEFENDING You took 516 damage and deflected 432!
DEFENDING You took 649 damage and deflected 289!
ATTACKING You dealt 823 damage to SCARY MONSTER and 398 was deflected!
In this example you take two rounds of defending for every 1 of attack. Hence getting hit by two monsters. But in the second round your AC is less effective ( i factored by about .66 threw in some randomness too ) because your not blocking, but your Armor still blocks a good amount of the blow.
Now had it been in the current system it would be like this
DEFENDING You took 1165 damage and deflected 432!
ATTACKING You dealt 823 damage to SCARY MONSTER and 398 was deflected!