First of all, I would never recommend doubling all of the vocals for an entire song. I think doubling should be used more sparingly, just for accenting certain words, sentences, or rhymic phrases. You can always layer a high scream and a lower scream or growl for some cool sounding stuff, too. But for doubling exact parts, I tend to turn them down about 3db each and pan them hard left and right. Just thow some kind of meter on the master fader, (being that you're a Pro Tools user, it probably came with the BF Essential Bridge Meter, I can't remember the name exactly), solo your vocal tracks, and adjust the volume of the doubled stuff so that its RMS level is the same as single tracked vocals. And yes, exact doubles always should be as spot on as possible, but with screamers I don't think it should be incredibly hard for them to scream the exact same thing twice.
I guarantee you that on the majority of CDs you own, there are doubled screaming parts that you never even noticed, just because they were performed really well and the levels were adjusted properly. Just listen on headphones and pay close attention!