Going to be putting together a war belt, have no other tactical gear or experience.  I'm looking at the Esstac KYWI 2+2 mag pouches.  What are the pros and cons of the pistol mags being stacked on the outside of the 556 mags vs all being in line?

I find the stacked pistol magazines a little difficult to purchase if a rifle mag is still indexed.
I prefer gear(like the bottom pic) that works for me, over retraining mechanics to use new stuff.
Not a big deal though, just my 2 cents.

I agree with first post.  

Perhaps more difficult to grab your mag with the other on top of it.  

Repeated training may eliminate that but meh.  

Flat-side by side for me.

No stacked.

YMMV

Pros:  less real estate taken up.

Cons: bulk.

Me, I hate stacked pouches anywhere.  On the belt, my arms and other things are bumping into them.

Slick as possible, for me.

Agreed

If your goal is as much ammo and gear as possible stack it (think infantry soldier) but if you are civilian think self defense you probably dont need that much ammo and would be bettered served with less weight and bulk. Couple rifle and pistol mags is probably sufficient.

On a belt I only prefer single stack.  Fully loaded pistol magazines weight a fair amount and the more you stack the more the weight will pull out away from you, especially if you're depending on MOLLE.

Stacked mags on your belt kinda sucks. You don't realize how much bulk they add until you find yourself always bumping them into things. I'd definitely recommend getting the in-line pouches.

Has anyone used a side by side one piece pouch like the one pictured in OP on a belt? I'm wondering if something like this is too stiff?

I've always heard that individual pouches allow more flexibility, but I've never used a big single setup like this.

Definitely. Go prone to find out how much stacked pouches really suck.

The benefit of stacked pouches is when you don't have enough room elsewhere on the belt.

Everyone's needs may be different, and this is just an opinion, but if you have enough stuff on your belt that you need to stack pouches to make room for it all, then I would go back and re-evaluate how much stuff you actually need on your belt.

Keep your belt restricted to your pistol holster, one or two extra pistol mags, mini IFAK, one rifle mag, and maybe a knife. Maybe also a dump pouch if you think you need it. Even all of this makes for a fairly cluttered belt.

If you're doing something where one extra rifle mag on your belt along with the mag in your rifle isn't enough ammo, then you probably ought to be wearing a plate carrier or chest rig too.

The bulk of the gear already gets in the way, I wouldn't want to negotiate anywhere close to tight with stacked mags.

I am in the minority in that I don’t run rifle mags on the belt. I like to keep it simple with handgun, handgun mags, tourniquet, and dump pouch.