I could only imagine how much faster they would level if they were panda monks who did the Zen pilgrimage every hour for the 50% Xp bonus on top of the BoAs they are wearing.
My guess is, they didn't choose pandas because they would be stuck in "Panda-land" for their first 8-10 levels... with no mailboxes... and therefore with no heirloom gear until they finished off all their intro quests. ... Also stuck at work now, can't watch the stream... but sounds like an interesting challenge. I'm hoping they are running this legit, and not like some of the people who were racing for world-first/realm-first L90 characters and had lots of guild assistance with downing mobs. Also - heard someone say they were on a PvP server... have they been ganked yet?? ... EDIT - I'll put my guess in as level 68 for both guys after 24 hours. Not L90, but still quite impressive.
Since this morning I've probably watched an hour of this stream all together. I've seen them encounter one person while questing, a Warlock of the same faction...he killed some named quest mob about 5 seconds before they got there...they were pretty pissed, lol.
To answer your other question, the two of them have played as a duo the whole time I've watched. No guild help or really any interaction with others at all.
OP stated they chose Hunter/Warlock for the ease of leveling with each class. -- McBrain#1430