My writing partner (Michael McNelly) is a friend since high school, so we've built up enough respect and trust to be able to criticize each other's stuff directly. Mike tends to do more plotting and I tend to do more prose-polishing, but we both work on both aspects.
The toughest part, I think, has been getting the pacing right… making sure the various twists and turns of the story balance between "character-driven verisimilitude" (which, let's face it, can get deadly dull if it's too "real") and "rock-'em sock-'em roller coaster" (which can feel contrived and cheesy).
It's fun to collaborate; my composing job has been good training in that way. Telling cool stories with friends, and getting paid to do it. Dream gig.
Here's how the book started: Mike and I were sitting around over beers one evening, lamenting some of the Bad Fantasy Fiction we'd recently read… and decided, why not step up to the plate instead of criticizing from the dugout? (…ouch. That's a terrible metaphor. Good thing I'm not a writer or anything.)
That conversation took place in 1997 (!). Turns out it's a tough thing to do, writing a big ol' fantasy saga. But I'm proud of what we've come up with. And the laughs along the way have been worth it on their own.
In terms of the iconic authors you mentioned… I will second what you said! I'd love to split my time between writing music and writing words. Leaves me just enough time for sleep and cooking dinners with my wife.