This achievement unlocks once both character have entered your party... Owlman becomes available after progressing to a certain point in the story and can be purchased for either 150k or 250k studs. Talon on the other hand comes from a side quest in Central Gotham... Speaking to the Johnny DC Info Point within Gotham will show you his map location. The side quest requires you to wear both the League of Assassins mask and body piece.. follow Talon to a truck and use a character with grapple to open the trunk and then step on the button. Once this is complete Talon can be purchased for either 150k or 250k studs.