Hmm...

I personally still feel that 4 shamans offer the best for PvE over any mixed class group. Case in point look at all the people boxing in PvE on this forum right now - everyone who has progressed the furthest/fastest is running tank + 4 shaman, and that setup generally has the least issues with each encounter. (Another case in point - tons of threads with people saying things like "Ok but how do you beat this guy without 4 shamans!??")

Anyway just my view, if you want to min max I still think 4 shamans is the best even before 3.0.8 (but after 3.0.8 our damage went so much ahead we obliterate things now) - even if you factor in other things you'll get with other classes that you can't get with 4 shamans, you also have to factor in that having more complex cast sequences and control will drop your effectiveness.

Now for those people who want mixed class setups because they enjoy it, or they just want more flavor, etc, then truly thats a great reason. For those that are min maxing...I don't really think you're gaining anything by having tank + 4 other classes over tank + 4 shamans. I can use myself as an example - I am in mediocre PvE gear at this point, a few epics from badges/heroics, and a couple crafted epics on each char, and then the rest is still blue quest gear, and I can blow through gundrak, DTK, UK and CoS heroics in 30 minutes each. I haven't seen someone with a mixed class setup that can do that yet (not saying there isn't one, just haven't seen it).

Now, back on topic (mostly) - as far as deciding between 1 or 2 shamans, I don't think it matters much, just going from 1-2 is probably not going to help you any more than just adding any other class instead of the second shaman.

Edit: Just wanted to add something. Someone said Shaman teams don't add anything you can't do better with mixed classes but I disagree. The way I see it, shaman teams add things you absolutely can't do with other classes but other classes don't really add anything important that you can't do with shamans. Quick fact check:

------------------------------------Shamans-only--------------Mixed classes
DD Spell Immunity---------------Yes (groundings)-----------No
Fear Immunity-------------------Yes (4x Tremor)------------Partial/No
Res/Ankh-------------------------Yes/Yes---------------------Yes/Maybe
Healing----------------------------Yes (4x)---------------------Yes (< 4x)
Maintenance Free AoE healing--Yes (Healing Stream x4)--No
Extra Free Mana Regen----------Yes (Mana Spring x4)------No/Partial
Bloodlust---------------------------Yes---------------------------Maybe
High resist bonuses--------------Yes (nature, fire, frost)----Maybe (paladin)
Poison/Disease/Curse cleanse--Yes---------------------------Maybe

These are the important things (for PvE at least, and a lot for PvP too) and as you can see the shaman have all of them, whereas mixed classes will only get you some, depending on how you mix them up. I didn't list the one big advantage of mixed classes such as HP/Mana buffs because they're actually fairly trivial in the grand scheme of things. Sure having a bit less threat or some extra hp or mana is good, but it hardly will make the difference between blasting through an instance and wiping, whereas Bloodlust, 400hp/s free aoe healing, Fear immunity, DD spell immunity, etc, can make or break an instance run, especially at lower gear levels. I would encourage anyone who has examples of things mixed groups can do that are absolutely essential that a 4 shaman team cannot do as I couldn't think of any when writing this list.