I think it's like this:
Option 1: Player A attacks Player B's unit with his 1st Infantry. The defender doesn't play a withdraw, commander or unit card. Player A then plays a commander card which allows him to have the 2nd Infantry join the attack. The base attack value is the sum of the 1st and 2nd Infantry attack strength. To this add the 1st Infantry's die(dice) roll (for the card that started the attack) and any dice rolls from additional unit cards (for either unit) that the attacker chooses to play.
Emphasis are my changes to the original text.
If I am wrong I hope someone corrects me as I would be playing the game wrong.