In order to be able to use Remmina, you need first to activate certain options of the "Desktop Sharing" screen (Dash -> Desktop Sharing) on both computers:

On the Desktop Sharing Preferences:

• Activate the option "Allow other users to view your desktop"
• Activate the option "Allow other users to control your desktop"

For security's sake, also:

• Activate the option "You must confirm each access to this machine"
• Activate the option "Require the user to enter this password" and provide a password

Once the configuration is done, you can simply open Remmina, create a new connection and specify the connection to the other computer using VNC (virtual network computing) protocol and using the password you used in the "Desktop Sharing" screen. Hint: You can ignore the field "user".

Regards