I don't have experience playing online, and I might be wrong. But in my opinion, no one would play like that in the 4th installment. Why attack from two sides? It means dividing the heroes and units into two armies. And what if the defender takes his undivided army and eliminates these two weakened armies one by one?
I'm saying this in the context of the diversity of strategies and tactics... it shouldn't be taken literally :) although such things only add to the game :) (for example, in Starcraft, a couple of High Templars can kill a bunch of Battlecruisers).
Now, a little off-topic.
Did you play Kohan? In my opinion, it's a fantastic game for online play.
There, units are squads of 4 to 8 people, and if even one soldier flees, the entire squad can be restored in the zone of their city.
On the other hand, if you exceed the squad limit (the more cities, the more squads), you can't build more units after they die...
And if you don't have enough money to feed them, the units will slowly start to die.
And there's also morale, and if the army fights for a long time without a clear victory, it retreats :) regardless of the player.
In general, the game is unique in terms of gameplay :)
So, I had a battle online, the opponent was pressing on my border city, the forces were equal... for about 20 minutes, we were constantly gathering armies and fighting near my city... he had an overall advantage, more units and resources... but he attacked this city with his entire army... I held him off for a long time (in the city, firstly, there are militia, and secondly, when the city is not under siege, its own units replenish their health), and he would have defeated me if I hadn't detached a couple of cavalry units and captured his cities and mines in the rear. He got bogged down at my city, and the advantage shifted to my side :)
I don't know if this is possible in Heroes... it turns out that online battles in Heroes boil down to trying to minimize losses in battles with neutrals for the final battle?
I don't have much experience with Heroes online myself :)