This is the EXACT campaign I'm running! Will respond more later when I have a computer handy. Or if I can find my own links in the Giantslayer forum.
Finding my own old, original thoughts on this:
Well, here goes!
Spoilers only for those interested in playing or being spoiled on both Giantslayer and IronFang Invasion.
Figuring in the time it takes me and my group to arrange our lives (all 40,40+ married with kids), getting to and through the 6 APs I have waiting on the shelf, while still going through my current campaign (which needs another 4 years), I got the brilliant notion to combine these two APs together...while using storylines from Storm King's Thunder as well! (I'm a Forgotten Realms mainstay fan; everything gets converted over to my Realms). Brilliant, right? Brilliant!
Don't worry, I laugh at myself also.
My want in combining these 2 is that both are such battle heavy, army invading, destroy mass numbers of enemies rich that I don't want to spend 2 campaigns worth 'doing the same thing.' If I make one large gestalt, I can cut out the parts I don't want to do (finding resources to get refugees to live in IF 1), and cut out things I simply don't care for (hunting down the horses stolen from the boat in GS 2 by a wyvern or drake-don't remember, -or dealing with the irate lover from GS 2 on that same boat trip as well), or flat out cutting chafe (ie, the very repetitive 'go kill this next stronghold of other giant-types!' that makes GS drag and get boring).
So, I'm tossing out the ideas I wish to use and change, and also get some other tips or ideas from those that have solidly played these, instead of reading them only like I have, about things you would definitely change or toss away-or even how to make them merge better.
I know I'd like to run my current campaign still face to face with all my friends, but this IG FS campaign I'd take to roll20 so in the times we don't play for 3 months, we could always just hop online on a Friday or Saturday when convenient for 2-4 hours and rend some enemies!
I'm thinking to start, I'd like to keep most of GS 1 'Battle of Bloodmarch Hill' intact. There will be ominous signs or portents of enemies on the move, the murder mystery within town, trying to figure out what the dead sheriff had discovered, wondering what all the white crosses were. But I also would need to utilize the town of Phaendar as a place, base of the mountain for Trunau, as a supply town. Make a place where the PCs and NPCs also have relatives or connections. Make Trunau really the 'last line of defense' against the wilds of the hogoblin tribes and giant forces that are out in Lastwall. Truneau protects Phaendar as much as Phaendar keeps Truneau operating.
So, the attack of GS 1 on Trunea happens as it does. From there, it would be mounting up a counter force to respond to Phaendar's fire beacon for aid. There would still be plenty of captives that Scarvinious' troops would have taken, so there's plenty to do and go out and about to rescue them still in the encounters that are listed in 'Trail of the Hunted.'
However, there still has to be the encounter within the tomb under Trunau for the giant's gear. Also, within 'Trail' there is another tomb that has some giant sized gear within the troglodyte section (which is an angle I'm totally doing away with. This can just be another tomb that is being searched for ancient giant gear and magic-which feeds into the storylines from Forgotten Realms of when, in ancient times, Giants and Dragons fought for supremacy of all the lands).
So, Scarvinious, is not only out to destroy Phaendar and Trunau, but he also has to find purported giant artifacts within burial mounds. His forces will be spread pretty thin at one point, which should be hinted at or dropped enough in clues for the PCs to want to make a quick strike force attack on his Red Jaw encampment...
And that should take care of a goodly amount of both volumes 1s.
It didn't make sense to me that in GS 2, you can just roll down the river to the Red Lake Fort. That's why for me, combining both Book 2s into fighting and freeing outposts and forts seems like a logical idea; all were identically attacked due to the Onyx Towers.
I've written this answer in 2 parts; I think the only STorm King's Thunder angle I'm keeping is that the ordning is off, which is why Volstun is doing what he is doing.
As for the Realms, apropos enough, this will take place on the north side of the Giantspire Mountains. This region is largely void, of well, anything. I'm putting this region as an area that was once under Damaran rule, and probably under Narfell rule a loooong time ago, but now is mostly just filled with frontiersmen and settlers-Damaran descent peoples' seem the perfect analog to the people of Trunau, and even Nimrathas. Half Orcs are tolerated here as well, I intend to have a good dwarven influence, and hell, I'm even sticking in 'Forge of Fury' from 3rd edition because my friends and I never even ran it, so I figure this will be a place where there can be more artifact/relic items, as well as a good living source base for knowledge of dwarf legends of the past.