Hot Key Net is probably the strongest of the free options.
You could also search for Auto Hot Key or Octopus.

HKN and AHK both use scripts.
Find one that does what you want, and go with it.

If you're willing to pay to box, I'll strongly recommend IS Boxer.
It is $10 for 3 months, $36 for a year, which isn't much considering the costs of 2 accounts per month.
IS Boxer is much easier to configure to do exactly as you want it to do.
There is strong support for it, especially on the IRC channel.

There is a 7-day free trial version, which doesn't hurt to check if its something you'd like to run.
Really, if 36 per year is too much of an investment, one month of subscriptions for two accounts is too much too.

My guide (linked in signature) is an overview to a lot of the options within IS Boxer.
To start, I'd just do the initial section which overviews the wizard.

That will get you, any key on 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, - or =.
Will broadcast to the other client, and they will both cast at the target of the active window.
With Jamba options, you can have the second toon auto follow the first, or on a hotkey you choose.
The wizard will get you up and running in a minute or so.

When you want to do more, or configure IS Boxer for your own needs, use the guide or videos as a reference point.
But also feel free to ask questions, here or on the IRC.