Of Ess: 15.9 MULTIPLE TARGETS
When casting a spell that can affect multiple targets, the caster can concentrate in order to “hold” the spell until it can be applied to the allowed number of targets. For instance, a spell user casting Deflections II may “hold” the spell until two missiles have been fired at the caster and deflected.
I also allow caster to "hold" a spell until the maximum number of targets. Casting Bladeturn II you could apply it to 2 attacks this round or 1 attack now and 1 attack later. Holding requires concentration, 50%Act. That can be a big sacrifice in combat.
This is risky, because you now have "declared" that next rnd you are maintaining the spell. 50%Act to hold and C burns exhaustion.
(I almost never use exhaustion, but I do consider it for extended actions; dashing, chopping wood and then being attacked, wearing heavy armor in the desert. This way, most people can't hold spells for very long.)
For my games, instant spells don't have to be declared at the beginning of your turn. We use instants as a reaction to events, mostly defensive and reflex magical training.
In my mind, the precedent here are subconscious spells. Those spells imply that a caster's mind, aura, magic essence, whatever, is always a bit active.
How would a caster live long enough to become powerful if they had to declare a bladeturn, when another spell (of their 1 spell/rnd limit) would have saved them? Your game may vary.
The duration on most of these spells is "--", no duration. The effects happen immediately, right then and there. As above, you can hold a spell (with C) until the max number of targets are affected.
I do like the idea that you have to cast it after your attacker has stated their intent and before their roll.
As a house rule, we also allow 1 instant and 1 standard spell to be cast in the same rnd. You can cast a spell and still have your reflexive magic work too.
(I could see an argument for requiring initiative, but I would not use it for our game.)