[Logician]

As for template modifications, you could search keyword "avatar" in the templates via Admin CP/Templates/Search to see which templates you need to edit. However to save you from a lot of editing I would suggest using the default variable for your multi-avatars and instead devise a method that would make it hold both avatars. (Hacking function getavatarurl in the admin/functions.php would be a good start) This will save you modifying a lot of templates.

As for scripts:
register.php => registering users assigned avatars here
member.php => for existing users.
admin/user.php => admin setting avatars for users.

are the firsts I can think of. If you dont apply the method I suggested above you need to change many other scripts which displays avatars for some reason (eg. admin/functions.php getpostbit template for displaying avatars in postbit etc.). To compile an exact list just search the keyword "avatar" in vb scripts.

What you intended to do is not an easy hack. Good luck..

PS. I'm sure u also know it but I'll say anyway: You can set vb to accept larger avatars and you can merge more than 1 graphics into one gif/jpeg with transparent bg and use it as your avatar which would give the same effect of having multi-avatars. Easiest way..