If you're following a guide like Kopp/Joana/Zygor, and have RAF, you can skip all of the collection quests and easily out-level the guide. RAF experience is so fast, you'll quickly reach a point where the guide is telling you to do whatever, but the mobs and quest are gray. As in, you'll have to skip sections of the guide and jump ahead to a more challenging/useful/rewarding portion at least a few times.

If you don't have RAF, follow the guide through as it presents the quests. Collection quests which aren't terribly painful can be done on all toons. Those that are painful, only complete on one toon, or skip entirely if they suck badly. Just switch which toon completes each collection quest so they stay roughly equal with each other. If the quest is easy on the toon doing it, the other can do it too. If the quest is normal, just finish it on the one toon, with the other toon doing the next collection. If the quest sucks even for one toon, then abandon it on both, but remember it is this toons turn for the next collection.

Once you reach outland, you can safely skip almost all collection quests again, because with the release of WotLK, the experience needed per level in Outland was reduced by roughly 30%, and the guides were written assuming you'd need 100% experience to level not 70%. So you'll level earlier then the guide assumes, if you do all the quests it suggests; and if you skip some quests here or there, you'll likely still be close to the expected leveling pace or more likely ahead.