General :: Sharing A Home Directory On CentOS Box?

To be clear: I searched and used the button above to find similar topics, but my solution wasn't there.

I was very busy with work the past months, so my quest to become a Linux Crack hasn't borne too many fruits yet So I am still a ...

So here is my problem:

I have 3 users on my CentOS VPS:

root
user1
user2

For security reasons I only log on via public key, root access is denied. Admin is user1. user2 is a regular user.

I want user1 to be able to additionally access user2's home directory, when I log in via SFTP using FileZilla, because occasionally I want to move files from user1's home directory to user2's home directory. But I need user2 to still be able to connect to his home directory via SFTP, too (everything I tried up to now always "breaks" his account).

How do I do that?

user1 is in the wheel group in order to be able to become root if necessary.
user2 is in the user2 group.

I tried adding user1 to the user2 group, but that alone doesn't seem to give him access to user2's home directory.

I know this must be about rights and permissions, maybe group permissions? I sit on it about 8 hours now and don't see no light at the end of the tunnel. When I think I got it I suddenly can't log in with user2 anymore (neither PUTTY nor FileZilla).