As mentioned before, spawning a second instance of SSH daemon running on a different port would be the easiest approach.
You may also want to use something like rssh (
rssh - restricted shell for scp/sftp) as the default shell and restrict all users to SFTP/SSH *ONLY*. You can even use rssh in both daemons, each one using its own copy of rssh.conf.
Keep in mind that rssh is not an official package and there are no pre-compiled packages available so you'll need to compile it yourself.
One benefit of this is that you won't need to mess with sshd_config and potentially break something else.
rssh is also compatible with older versions of SSH so it should work just fine with "ancient" versions.