I always appreciate the feedback, Markc.
So, one possible solution could be...
If casters want to be able to cast one instant and one normal OR one normal and then one instant they must state it at declaration of actions. Even if they aren't exactly sure if their Bladeturn is necessary. Could a spell also be cast and then the caster wait for a suitable target... next round? I wouldn't allow this with instants. Just normal spells.
How long can a caster wait? Is that considered concentration? It might not be if the caster has to observe to find a suitable target of the spell. They wouldn't have much %Act left. 50% concentration and 40% at most for a -30 Observation, IIRC.
This is beginning to mirror a conversation held in the HARP forum recently.
Here is a link to a post I made in that thread -- http://www.ironcrown.com/ICEforums/index.php?topic=9384.20;msg=121517
Depending upon how you are handling initiative and percentage activities in the game, the results of trying to cast a instantaneous spell and a regular spell should follow from that.
The basic thing to remember is that a character will NEVER be able to cast an instantaneous spell WHILE casting another spell - period.
Personally, I would say that "Waiting for somebody else to take action, does take a character's activity", so that means that if a spell user is waiting for an archer to fire a bow, then the spell user is using up his activity waiting for the missile to go off.
As Lordmiller points out, the release of the arrow need not be at the end of that 50%, however, it it isn't, then you need to, as a GM, designate a specific activity percentage point in which it does, and then apply that consistently for all (PCs and NPCs/Monsters).
The reason that I say the above is that the wording of the Deflections I spell (RMSS Spell Law, page 89) clearly uses language that indicates that the spell is already in flight.
And unlike melee, in which the attack roll is considered to be either "the best strike" or "the culmination of multiple lesser strikes" (and thus are actually happening over the entirety of the percentage activity for a melee attack), a missile attack is one shot, one hit/miss.
To me, this says, the spell, Deflections I, cannot be cast until the missile leaves the weapon, and thus makes it something you cannot really declare beforehand. At least, I wouldn't allow it to be declared before the missile was fired (and yes, that means it is super-fast to cast, and as LM suggests, the 20% activity for casting it could/would be considered recovery, after the fact).
(Bladeturn, on the other hand, I would allow to be declared, because of the definition of what constitutes a melee attack, as mentioned above, thus that spell actually applies to the "roll" made for the attack, not each, individual strike.)