The twice as many Shermans would still do four times as much damage to anything (else) they can damage, before being defeated.
The simplest fix is to the encounter building rules/guides, letting the DMs know NOT to use twice-as-many but half-as-weak, or half-as-many but twice-as-strong, monsters.
Lanchesters equations or The Salvo Equations. If an enemy force has twice as many numbers, and all of them can attack, then they will do four times as much damage before they are defeated.