process user differently using ssh key
I'm not sure if you want different chroots, but the trick is usually to identify the users in authorized_keys. Take this example from Gitolite:
command="/usr/share/gitolite/gl-auth-command john",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty ssh-rsa AAAAB3NzaC1yc2EAAAA...
The gl-auth-command then knows who it is, even though everbody signs in with user git.