I'd go against switching toons, focus on one. Otherwise you'll end up in doing same areas and, especially, dungeons twice over in quick succession once you hit Outland. Also lets your first 90 to get access to dailies/mogu coins/palace keys etc. when you do other one.
Once you hit 90, you'll still need to grind rep. Take one toon and grind rep until you can buy the commendation that gives you extra rep. Then, when your other toons hit quests for those factions, you'll get a head start on your alts' rep grinds. Remember that 5.3 is supposed to be substantially reducing the XP requirement to hit 90, so try to save most of your leveling through Pandaria until after that change hits. Other changes in 5.3 should make it easier to gear your toons too.