my 2 cents:
I find it easiest to level by zone. Sure having your 2ndary close to your primary is useful. But it's not THAT useful. You can get by almost as well if your secondary is level 20 than level 25. As a completionist, i like to go through a zone, finish it, then move on, and the game allows you to do this relatively well. For levels 1-30 you can do a number of things, my preference is to level 1-20 through howling mountains (which can get 2 classes up to 10, if i'm an elf i'll do through 10 in elf then through 10 HM), then silverspring 10-20 on both (you have to do EVERY quest though, i find it worth it and it gets you the FA set). After this your 20/20, you can get your 3rd class and start going through Coast of Oppurtunity, getting your elite skills there at 20/20/20.
At this point choose one class and take it through Aslan until 30, then level your secondary through Dragonfang (or through dailies, up to you) and your 3rd through coast to 30 then get your 25s and 30s elites. After this it's a mix of dust devil, ravenfell, weeping coast and dailies/coast for other 2 classes up to high levels (and then the ch3-4 content to reach 67, but thats pretty far away).
Note if you dont want to be a total completionist like me, you can easily go through the Steppes on your 2ndary to get to 20, and just take one class through Silverspring, its purely preference.