The key issue is that most quest guides for fast power-leveling, either free ones like Jame's for Alliance or paying ones like Joana/Mancow for Horde or Brian Knopp for Alliance are designed for pure solo leveling.

They even ignore on purpose instance quests since 2.3.

I haven't found any guide dedicated for Multi-boxers, specifically avoiding collection quests.
Probably because the "market" is too limited to expect any decent ROI for the writer.

So far, I've found that the usual instance runs "in a loop" is best until lvl 50 (ie. for Alliance : Deamines -> Stockades -> SM serie -> Uldaman -> Zul Farrak).

Then you may choose to mix both open-world quests and some LBRS runs until 60 (many advocate for getting to 60 before entering Outland, especially on PvP servers).

/salute