Do spells stack?

If two elementalists cast something like Savannah Heat on the same target simultaneously, or if one casts one second after the other, in either situation will the target be affected by two ongoing Savannah Heats at the same time?

Yes, though 2 the same hexes on a target won't stack, some with conditions.

Savannah heat is an AoE spell, technically it effects the area not the person. Spells that target a person with a duration (such and enchantments or hexes) do not stack.

correct me if im wrong but.....
conditions,hexes, armor enchancing buffs(above +24), and enchantments dont stack. anything else is stackable as far as damage and healing is concerned or else there would be no such thing as a spike.

Actualy there would still be spikes, ive seen many, many spikes where there are no multiple copies of spells/skills used.

yeah... XP totally

As an Echo nuker, I can tell you from experience (unless I've been completely confused) that yes, basic damage spells stack, whether instant or DoT. However, most other things do not stack when two identical versions are present:
-Identical enchantments don't stack; re-casting will instead renew the duration.
-Identical hexes don't stack; see above.
-Identical hexes don't stack; see above.
In addition, some non-identical things don't stack:
-Degen/regen don't stack above 10 pips, effectively (though if you have something doing 11 pips of regen and 2 pips of degen you will end up with 9 pips of regen - it does factor in in these cases)
-Armor buffs don't stack over +25 OVER MULTIPLE SKILLS. However, if one skill raises the armor over that amount (Save Yourselves! is a good example), the full increase will apply.
-Speed buffs/debuffs don't stack over 33% OVER MULTIPLE SKILLS; see above.
-Stances don't stack; one will overwrite the other.
-Weapon Spells don't stack; see above.
-Item Spells don't stack; see above.
-Theoretically, Forms don't stack, but the only time this would apply is when you're capping skills as all Forms are elite.

...and I think that's everything.

And to add on to that - glyphs don't stack. One will overwrite the other.

Another addition to the list: knockdowns don't stack. If a knocked-down target would be knocked down again, the second KD is completely ignored (and will not add to the duration of the first KD).

If I recall correctly, Meteor Shower when it's affected by +1 to knockdown duration from Earthbind is iffy - as it KDs precisely every 3 seconds for 3 seconds, subsequent KDs sometimes do trigger and sometimes don't.

Yet if the caster would recieve a benefit form hexing a foe, the 'benefits' do stack.

Like if three people all cast [Assassin's Promise] on the same foe, and that foe dies, all three people will recieve the Energy gain and Skill recharge.

Some Hexes do actually stack, like Asuran Scan or Assassin's Promise.

@glacialphoenix: Oh, thanks, I forgot that one. Rarely use glyphs...

@Arduinna and Neo: Yeah, so I should clarify: direct negative effects from hexes on foes don't stack. But hexes that indirectly affect a foe by helping the caster do stack on a single target, so as has been said, if 3 people cast, say, Asuran Scan on one foe, each one does +damage against that foe.

Stacking is sorta kinda iffy. Some things do, some things don't, some things do - but not fully...

I've seen explanations of what happens when 2 or 3 Players/Heroes use Monk Bond Enchantments on the same Party Member - again, it will only show one copy of the Bond on the Party Member, but the Players/Heroes that have cast will still show that they are maintaining each of their Bonds - and the effects will be spread.

So yea, sorta kinda iffy on a few...