Here you have a Photoshop header Template for Twitter header image for accounts with multiple members. You just have to open the PSD file and put your background image (1252 x 626, 2:1 aspect) and your avatar images for each member.
The template for the header is fixed for 4 avatars plus the central one, which is the avatar of the Twitter account.
Thanks for visiting and happy use!
You can download the template here.