there is a scenario where if you have *both* ropes running through a groove together, they have a tendency to draw each other into that groove or flake
eg the last pitch of Diedre. Â I have witnessed a rope-management debacle here. Â Half ropes can twist over themselves like mating Anacondas, quite reluctant to let go their passionate embrace. Â Ugh.
Employ the "rope flick." Â Note what you're climbing past, and (for example) if you are moving left after a right-facing corner, pull your rope out of it. Â If you've ever been given a bad belay, you'll know what I mean... you need to tug the rope from between your legs in order to move. Â On long routes the rope drag can be an issue regardless the quality of the belay, so get in the habit of testing it as you move. Â If it's getting worse, stop and build an anchor, and belay there. Â You'll save time. Â
Personally, I have climbed Moby Grape with half ropes and had a wonderful time. Â For example, the pitch above the Triangle Roof requires that you clip one rope for the first fifty feet or so, and then use your other rope to protect you. Â You may be tempted to simply alternate clips... and this will bugger you.
An even better example is the pendulum pitch on Vertigo. Â Clip only your left rope up to the pendulum bolt, lower down & swing over, and then clip only your right-hand rope. Â Drag will be a at a minimum. Â In order to climb this pitch with a single rope, after the pendulum you need to climb 20-30 feet before placing gear, or your rope will be making a hard ninety-degree turn, a rope-drag nightmare.
Hope this helps.