I don't know if I've missed this somewhere, but can someone answer this...
You have (say) Ward Against Heat and Flame active,
This adds +15 to your soak (IIRC)
(assume no other soak)
Now what happens if someone hits you with Two multicast fire spells that do 15 damage each...???
Do you take:
The Ward works against each spell individually..
The Ward works against both spells together because they arrived together...
I'll pipe in here with a boring "I agree with Furian and Caribet" answer and add that a spell like Pillum of Fire with a listed damage of +15 in fact does simple die +15 damage not simply 15 damage. You probably knew that already but it isn't really promenantly displayed in the rules.
Something my troupe looked and looked for was if armor soak helped against spell damage (such as fire or a water blast, or acid). As there are no rules stating anything other than armor gives bonuses to soak, we play that it does. Sometimes I rules armor is damaged or actually becomes so hot it causes its soak damage after a fire blast, but mostly I don't bother. Stuff like that is cool as spice, but messing with it too much can really bog a game down.
I would also say that each is treated seperately. Mytake on it is that even if they are multicast, they aren't cast at the same time and therefore aren't hitting at the same moment. just as if two seperate people were casting spells at you in the same initiative order they aren't both hitting at the same time.