keyboard - Synergy: how to forward same keystroke to client and ALSO to server?

I've installed synergy on two different machines... now I need to type one key in the server in order to obtain an action both in server and client (i.e. left and right arrows to control two different powerpoint presentation simultanously on the 2 machines).
I've configured the server but when I press the key in the server the action is executed only in the client.
Where is my fault?

Thanks!