Typically yes, there are. I don't recall the Hardy Boys books enough to be sure, but generally the reason for using a Main Crew instead of a Main Character (both referred to as MC, see how clever?) is to give the Crew members specialties; not keep them the same.
This can be more plausible; you don't have a single person that can do everything, and it opens plot possibilities that don't exist with a single character. One can get hurt, or trapped, and the other can save them. It opens up the possibility of a member betraying the Crew. Or a ditz good with a sword inadvertently letting key information slip out.
They not only have to get the mission done, but save each other along the way, and there are things that teamwork allows that cannot be done by a single person. One creates a distraction while the other sneaks in behind it.
When you watch Ocean's 11, everybody has a different specialty (well not really, there are only 7 or 8 distinct jobs to do. You don't need the brothers for radio control, you don't need a separate guy to buy the cars, that could have been done better by a radio control guy, etc).
The Avengers each have different strengths.
Even if the Hardy Boys were basically the same (I don't recall), two is better than one: You can't stand on your own shoulders, or throw yourself a branch to climb out of a pit.
I would suggest different character development for Crew members. You can make it just about numbers, two brothers are better than one. But even then, it is more plausible if one is mental and one is physical, give each of them distinct strengths and weaknesses.
The value of a team is synergy. They can do more together than the sum of the things they can do alone.
An added value of the team can be love and loyalty, to a "fault", that is not a fault at all in our human tribal eyes: Despite our feuding I will save my crew member even if I have to let the thoroughly captured villain go free. I will risk my life to save that reckless idiot. This can be setback in the plot, the team's plan is ruined, but in the eyes of the audience a positive emotional development, and drama. The hero did the right thing saving his crew member, even though it cost them dearly, and now they must regroup and find another way to take down the villain. And of course, the saved crew member is the one that gets the insight into exactly how to do that...
Crew members can also be on separate emotional arcs. One is up when the other is down; they can provide comfort and sympathy and affirmation.
The point of writing a crew is to provide variety and show the human side of how teamwork really works. We are not just interchangeable ants or bees where no given individual really matters. We all have emotions and individual personalities, we stay and work together and partner up for the synergy, it is our different strengths and weaknesses that combine, in a crew, to compensate for individual weaknesses with the strengths of others, and create a more perfect, more competent whole, with many strengths and few weaknesses.
The orchestra is not all just pianists.
