What happens if I must attack with two creatures, but am only allowed to attack with one?
You have to declare one of the Juggernauts as attackers. You cannot declare both of them as attackers. The important rules that you missed are under 508.1:


508.1c The active player checks each creature he or she controls to see whether it's affected by any restrictions (effects that say a creature can't attack, or that it can't attack unless some condition is met). If any restrictions are being disobeyed, the declaration of attackers is illegal.

508.1d The active player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must attack, or that it must attack if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of attackers is illegal. If a creature can't attack unless a player pays a cost, that player is not required to pay that cost, even if attacking with that creature would increase the number of requirements being obeyed.



If you declare both Juggernauts as attackers, it would violate the Dueling Grounds restriction, so it would be illegal per 508.1c. If you declare neither as an attacker, you are following 0 of the 2 relevant requirements, so it would be illegal per 508.1d.
However, if you declare one of the Juggernauts as an attacker, then no restrictions are disobeyed, so it passes 508.1c, and 1 requirement is obeyed, which is the maximum number that could be obeyed with Dueling Grounds on the battlefield. That makes it a legal attack.
It's important to remember that when the rules say that a choice is illegal, it doesn't mean &quot;you should go back and make a choice&quot;; it means &quot;you never should have made this choice&quot;. Rule 717 is there to handle the case where you broke a rule and the game state is fixable. There is never a case where you must make an illegal choice.