Shaman Monk duo, but only one fungi
 

So, who should wear the fungi for the fastest leveling?

Monk. in this exact situation at the moment and find that the shammy is normally okay for mana/health. We only just hit 24 though so things might change.

I see. We havent really got started yet. I have some cathing up to do before we can really get going.

The shaman can more or less just haste and regen the fungi monk and then go back to AFKing while the monk exps for both of them.

Monk. No question.

Agreed - monk is the proper choice here

monk as he will be tanking. Shaman can sit on his butt and canni only when needed.
Shaman should just keep stats, hp, regen and haste up on monk. Then sit back down and wait until the monk needs buffs or a heal.

slow on mob also? or too much threat generation?

Don't even bother. Just save your mana for occasional heals, a buffed fungi monk will dirt nap everything.