it worked! basically I couldn't Xforward from MAC because X11Forwarding wasn't set... maybe when I did it I didn't restart sshd_config.
Now everything works, in both directions: either I can 'remote control' one computer from the other via ssh session (by setting the proper DISPLAY settings) and I can access via ssh -X programs from remote machine to the one I use.
I'm quite happy....
Thanks again for your help, I really appreciate it