You can already pack your brothers tightly with designated “tanks” protecting softer targets, maybe the geometry isn’t as pleasing as a circle but it serves the same function.
Doubling the amount of soldiers on the field would send the game’s balance haywire. It also goes against the basic premise of running a company where each brother’s survival is meaningful and every move in battle matters. Plus, as the units in battle move one at a time, it would only hasten the arrival of the common mid-late game problem of bigger battles dragging out unbearably.