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.
I really don't think that's a problem - if you're sending poorly equipped soldiers with 10 attack each against some godlike creature with 100 defense, sure you won't even scratch it 90% of the time, but isn't that the way it should be in such an extreme situation? Practically speaking, it's rare to get single units with 100 defense*, and it's much more common to get single units over 10 attack, so that's taking a rare and extreme case and complaining that the results are.. well, extreme. Really that's exactly the result you should get, given unusually small attack vs. unusually large defense.
[*Edit: or perhaps not so rare, haven't encountered the 891 defense Snathi myself.. jesus, that's ridiculous. Clearly some other balancing issues going on here My point still is, though, that an enemy with defense 10x one of your soldiers' attack should be difficult to kill, not something easily solved by training a stack of 10 soldiers.]
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.
Not necessarily - pooling health together and treating the squad as a single unit for defensive purposes is ok, the key is not stacking defense values. That is to say, a squad of 10 soldiers with 5 hp and 10 defense each should act as a single 50 hp/10 defense unit when it gets attacked. You could think of the first 5 hp as the "first" soldier defending with 10 def, and at 45/50 he's incapacitated and the second soldier is now defending also with 10 def, but that distinction is abstracted away - in gameplay terms a single 50 hp/10 def unit has the same effect. The only difference is whether this unit loses attacks as it loses health - at 45/50 do you stop getting that 10th attack from the soldier you've lost, and get 9 attacks instead? Or just give it 10 separate attacks regardless of health? I'd favor the latter, it's simpler and gives you a reason to still want stacks of units. But either way, it's a massive improvement over the stacking attack/defense values that lead to 10 individuals forming a 50 health/100 defense/100 attack ubersoldier that is exponentially more powerful than the sum of its parts.