Ah, you will need key-broadcasting software if you plan on hitting 1 button and both accounts/toons attacking.

But if you are content to tab back and forth, you can set up macros on your second account to make it a little quicker.

/cast [target=party1target] Lightening Bolt

There are a few free programs you can try out if you don't want to spend the $20 on Keyclone.
While I haven't tested Octopus or AutoHotKey myself, I know Keyclone will get you up and running pretty quickly and easily.