I've mentioned this in other posts but a hardware way of doing this which is fairly inexpensive is using a device like a Nostromo N52 or a Logitech G15 keyboard, both which can be programmed with macros to ALT-TAB for you in addition to sending a keystroke to the other window, all with one keystroke.

Just have it do an ALT-TAB, short delay to allow screen to swap, keystroke to your second character, then ALT-TAB back to your main.

Be careful when using this type of device that you DON'T use delays that try to wait for cooldowns to expire (even the short global cooldown) in order to send multiple commands with one keystroke. This can reportedly be detected by Warden and may look like a Bot and can get your account banned.

If/when you move on to multiple computers and more than 2 boxes, there are better options. But this is not bad with one computer playing two characters.