While I'm all for making the groups attack multiple times it's going to take a lot more than splitting the attacks to balance it. A few scenarios to consider.
You've got a group of 10 guys with 10 attack fighting a dragon with 100 defense and 100 attack. The formula I keep seeing posted for combat is something like.
Attack Roll - Defense Roll = damage if > 0 else "Miss"
Currently you'd have 1-100 vs 1-100
In the new system you'd have 1-10 vs 1-100
Chances are that group of units would never hit the dragon, 90% of the outcomes for the dragon defense roll ends in a miss for the infantry. In the current system they are on equal footing for this scenario.
Now face the other direction, the dragon is attacking the group of 10 guys, currently it has the ability to kill them all in one attack. With the proposed system they'd simply die one at a time, lots of overkill for the dragon in melee. What took 1-2 turns before took 10 this time, and the dragon was never in any danger whatsoever. Shouldn't a dragon be able to hit more than one unit at a time anyway? tailswipes over large areas, breathing fire, etc etc.
In order to make this work you'd need to implement a system that had target limits for attacks based on the size of the unit, rework the attack/defense system and implement a damage/armor system as well. Example:
Base attack and defense of every creature in the world is 100, Modifiers include race/size/abilities/items/spells/stats etc etc...
Attack 150 (50 from size)
Defense 50 (-50 due to size)
Armor 10 (+10 from ancient dragon scale)
Melee: 15 Physical Damage Max Targets (3)
Fire Breath: 8 Fire Damage Limited Use Max Targets (10)
Squad (6) of Dragonslayers
Attack 110 (10 from amulet of accuracy)
Defense 85 ( -25 from heavy armor +10 from mount)
Armor 7 (5 vs Fire) (7 from full masterwork dragonslayer Plate)
Melee: 12 Physical Damage Max Targets (1)
Charge: Granted by mount melee damage +5 must cross 3 tiles before attack to execute
10 hp each
Combat would play out similar to this:
110 vs 10 (Critical Hit 150% damage) 25.5 damage vs 10 armor 15.5 damage
82 vs 36 (Hit) 17 vs 10 = 7 damage
98 vs 45 (Hit) 17 vs 10 = 7 damage
76 vs 20 (Hit) 17 vs 10 = 7 damage
56 vs 34 (Hit) 17 vs 10 = 7 damage
20 vs 50 (Miss....yes he missed a giant dragon) Scold him or something?
43.5 damage applied to the dragon
Retaliation by dragon for the attack is melee:
Melee gets 3 targets due to size so the rolls go like this:
150 vs 60 (Critical hit 150% damage) 18 vs 7 armor 11 damage
120 vs 72 (Hit) 12 vs 7 = 5 damage
73 vs 85 (Miss)
Dragonslayer I dies
Dragonslayer II is at 50%
and Dragonslayer III wet himself
That's just a minor example of how a combat would go, all the numbers need to be tweaked and whatnot but the general idea remains.
Attack/Defense for hit/miss only
Damage vs Damage resistance
Modifiers due to size/spells/special abilities/racial features/stats
Str = Bonus damage/attack
Dex = Bonus defense
Con = HP/slight armor bonus
Int/Wis = Elemental/spell resistance
Cha = ...... I don't know