You'll need some form of multiboxing software. This is what we use to send keystrokes to all, or specified ones of our game clients. There are a number of solutions available, so how specifically you would achieve this is down to the software you end up using.

Personally I would recommend ISBoxer, if you are willing to pay for the software. No other options comes close to it in.

As far as free options, I've only used Hotkeynet myself, which still works as far as I know, although it requires some tweaks that are not documented on the site due to it being out of development and the game having changed slightly since. Autohotkey probably still works as well, but I have no personal experience with it.

So, start by looking at software solutions and go from there.