Need clarification on Legendary Characters

Does the no more than 1 legendary character in a party rule apply only to the same copy of the legendary character, or to all legendary characters?

In other words, I know that I cannot have 2 Archmage Arugal or 2 Warchief Thrall in the same party. But can I have Warchief Thrall with Archmage Arugal (since they are different legendaries)?

You can only have one legendary in your party. So no, you cannot have Warchief Thrall and Archmage Arugal in the same party.