For what it is worth, I also cannot see that anywhere in the rules multiple attackers can contact a single defender frontally. The only case where the rules seem to allow it would be where a single attacker has moved into frontal contact with two defenders, and further attackers can contact the ends of the defending units not in contact with the initial attacker.
However, there is a tantalising suggestion that things can be different in the Tips and Tactics download. In the second paragraph on the Ottomans, it says "Your numbers are such that you can land 2:1 frontal attacks, plus flanking bonuses ..."
Quite frankly, we play it that as long as both attackers have the defender "to the front" and the charge is otherwise valid, then both attackers can contact the defender. Both attackers move together, not one after the other.