There isnt really a fix for it, I had/have the same problem youll quickly learn to let go of keys before switching windows. The problem is you're switching windows and WoW is still registering the keypress on the old window, Ive had ALOT of aggro fun form it but as a Warr/Sham it was easily taken care of.

As for 2 melee, I did the same thing as you Warr/(melee-healer) it isnt that bad, just make an assist macro I use:
/assist focus
/startattack [target=focustarget]

Ensures my slave(sham) is in melee mode and I just have qbound to 1, where that is for warriors charge. Basically charge warrior and sham follows in attack mode, the most annoying thing you will notice is the slave generally has bad vision of the target Id say 60/40 for the target being in LoS, with some manuvering it can be placed in LoS with ease. The paladin will definately be mainly a heal/buff bot, but I found in AoE with shaman I could lay down magma and assist in aoe dps etc, guess consecrate would be the same for you. I leveled as resto, which I sort of regret granted it allowed my warr to tank multiple mobs in crap gear with ease, but I am now respecing to a enhance/resto mix

Gluck

If ya have anyother questions Id be glad to help, I just hit 60 so have a bit of experiance for double melee under my belt 8)