Blizzard and Tempest may be cast by different characters. Only one character has to have Spell Might activated. The mage with Spell Might must cast second to create the spell combination.
Storm of the Century does not include the Blizzard effects, it is basically a scaled up version of Tempest.
The additional 100 mana that is drained is not required as the cost of the spell – therefore, if after casting all three spells, you have less than 100 mana, your remaining mana will still be drained, you will have 0 mana, and no further penalties (i.e. penalty to mana regeneration) are applied.
Having Blood Magic active does not change the additional cost – it will still drain mana, not health.
Spell Might remains in effect unless all mana is drained. Even if the party member offsets Spell Might's mana regeneration penalty with gear and talents, the sustainable will deactivate if and only if all mana is drained.
Storm of the Century's radius is double that of either of the component spells – therefore, make sure that there is plenty of distance between the storm and your party if playing on a difficulty where friendly fire is active.
Storm of the Century usually results in fatal cases of friendly fire to any party members caught in the area. If this occurs it may be useful to cast Glyph of Neutralization on the party members corpse as the member will most likely be hit (and killed) again when recovering once hostile activity has ended, resulting in more injuries.
Since the component spells of Storm of the Century can be cast outside of combat, one tactic is to Hold your party, cast spell might, cast the first component (usually Blizzard for the freezing effect), then cast the last component and move the spell caster and possibly the rest of the party back out of range. This will force affected enemies to attempt to run through the storm to reach the party, allowing the storm to do damage while the enemy crosses its area of effect. Note that it will affect killable NPCs.
Storm of the Century affects enemies through walls. So, for instance, the spell can use a closed door or the center of a hallway as the target. Storm of the Century will then damage enemies in range behind the doors and walls if the encounter does not require dialog or a cutscene to begin. To further exploit this, send a stealthed companion such as Zevran (or yourself, if you are playing as a rogue and has Stealth talent) to open doors and spot the enemies, retreat while still in stealth, then safely cast this spell while the enemies are still clueless. For added fun and damage, put a Glyph of Repulsion at the threshold right after casting Storm of the Century, and watch helpless enemies get tossed back into the storm when they try to run at you.