Unlike in previous expansions, RoK is not exactly duo friendly.
Essentially, it's been designed to be soloable to 80 through easy solo questlines in each zone. If you roll off the boat at 70.0 and do almost every quest you'll finish the last questline at 80.
There are only a handful of heroic quests along the way. With few exceptions these fall into two categories, those which are soloable by most classes, and those which require an actual group.
There aren't really any overland heroic mobs to speak of, there is no grinding.
The instances/dungeons are probably all too difficult for a duo, with the possible exception of Karnor's once it's greened out.
To sum up, you might want to duo all the quests together as it will save time and might even cut the tedium, but it's definitely not necessary so use whatever class you enjoy playing more.