[Santtu]

Control the tanking character (enh shaman) with your mouse, with your eyes on the tank. Associate easy-to-use general helping keys for the resto druid, who follows you. Occasionally the druid might draw aggro, so make a quick button for the druid to HoT himself, or do something else helpful. Make macros to heal your tank (shaman) by name, and to restore your killing target after. The druid should do moonfire and other spells to dps, but occasionally throw the heal.

The shaman and the druid should kill the same target; there is some lag between you changing the target with your lead character (shaman) and for the follower to recognize the changed target. In any case, there's no need for the druid to almost ever target any other monster than the one your tank is targeting; so you can make all the druid damage and harmful spells of the form,

/cast moonfire at party1target

where the party1 refers to shaman, and party1target to his target.

Make the same keys for both of them to drink, eat and sit.

Use software to copy the keypresses from one keyboard to both windows. Keyclone costs 20$ per copy and does a decent job. I heard there are free alternatives.

Put minimal graphics settings on both of your windows, but have them at least at 600x480 or so. There's no such option available; the smallest is 800x600. Still, the game gets faster if you enable resizing, and manually drag the window to a smaller size. I would never recommend using the main view at anything less then 800x600, though. You can disable the sound from the secondary game for slight speed boost.

I'd advise to spend most of the visible button slots for your shaman's abilities, and less for the druid, with a ratio of 1/3 or so. You can combine some similar kinds of actions for the same buttons, saving space and making it easier to work.