Unsure about a thing while trying to boost caster level. . .

[Shadowhunter]

Alright ladies, gentlemen and anyone else preferring to be defined by another term. I've recently taken to update a few of my old character builds and as I was poking at my Sneaky Spellcaster (Rogue 1/Diviner 5/Unseen Seer 10/Incantatrix 4 if anyone cares) trying to get the best Loresong and Hunter's Eye I could manage without consumptive field shenanigans, I became unsure how the following works:

Elder Giant Magic
You have learned a technique developed by ancient giant spellcasters, allowing you to channel additional power into your spells.
Prerequisite
Spellcraft 9 ranks,
Benefit
When casting a spell with a casting time of no more than 1 round, you can increase the casting time by up to 3 rounds. For each additional round that you extend the casting time, your effective caster level is increased by 1. This increase affects all the variables that depend upon caster level, including checks lo overcome spell resistance. Using this power is not without danger. Each round of increased casting lime, you must succeed on a Concentration cheek ( DC 15 + spell level + 1 for each previous check) or lose the spell. You cannot lake 10 on these checks.
Special
Creatures of the giant type can also use this feat to improve spell-like abilities, and they gain a + 4 competence bonus on Concentration cheeks made to use the feat.

SPELL ENHANCER
Transmutation
Level: Sorcerer/wizard 4
Components: V
Casting Time: 1 swift action
Range: Personal
Target: You
Duration: 1 round
You utter an ancient word tied to the fundamental principles of magic and immediately begin casting another spell.
This spell enhances the next spell you cast, making it more difficult for targets to resist. The next spell you cast this round is cast at +2 caster level, and its save DC increases by 1.

Do you need to cast Spell Enhancer during the turn you finish casting a spell, or on the turn where you start to cast a spell? Because in one of these scenarios, you can "Cast Spell Enhancer, start casting Other Spell. Prolong OS by 3 rounds. Cast OS at CL +5" and in the other one you cannot.

Which one should it be? I keep going between both options and my sleepy mind decided to outsource the whole issue to the braintrust. Any input would be kind.

[geniussavant]

Assuming I'm not missing something,  which is a possibility,  you would have to cast spell enhancer on the round immediately before or the same round as you cast the spell you want it to effect dye to the 1 round duration.  Now,  aiming you wanted to combine it with elder giant magic, things either get a bit complicated due to the single round duration, or really simple. This is because the verbiage of spell enhancer specifically states functioning on the next spell you "cast" and only having a one round duration.  Technically using EGM means that the spell wouldn't be cast when spell enhancer expires but still in the process of casting

Now, if I were Dming, I'd probably make a house rule and allow it to function. I can't really think of anything that the extra +2 Cl would break that I allow anyways. 

[Shadowhunter]

Thank you geniussavant 

Practically speaking, it'd be up to the DM I'd be playing with and this isn't a build I intend to throw at your average DM in the first place. So for now, I'm just going to pretend I have the more generous DM in my head saying "this'll work the way you want it to".

As an aside....metamagic feats don't have a spellcasting prerequisite. That's a bit odd, but convenient for someone who picks his first level as Rogue and is a bit feat-starved.
Hello Flaws, nice to see you. Oh well, I guess I can fluff that as a natural talent for learning spellcasting that he discovered, just like the Spellgifted (Divination) trait...