"I think we're gonna need a bigger box!"
If it's not too late, change out the 4x4 box that will have the GFCI receps for a 4-11/16" box, with appropriate domed cover or 2-gang mud ring. Two GFCIs in a 4x4 will be a very tight fit with almost zero side room... but 4-11/16" are a comfortable fit.
No, we're definitely gonna need a bigger box.
I now realize you're bring six 12/2 cables into the box. And also having 2 GFCI receps there. Aside from the GFCI-crowd problem, which you will find heinous, you also have a statutory (by the law) over-fill problem. That is far too much for a 4x4 box. Let's crunch the numbers.
- Hot and neutral wires entering the box = 12
- Grounds entering the box = 6, but you get 4 for the price of 1", so 2.
- For all cable clamps together, 1 count
- 2 counts per receptacle, so 4
That is a total of 19 counts. Uh oh. Now we multiply that by 2.25 cubic inches for #12 wire. Result is 42.75 cubic inches. This is way too big for a 4x4 box, even a deep one, which is 30 cu.in. A 4-11/16" box is 42 cubic inches, and presumably you'll have a mud ring or domed cover that will get you over the top.
The ground wires go to the metal box first!
They do not ground to the receps. Or to be more precise, cable grounds must be attached to the metal box first. If you remove all receps, ground to the metal box cannot be interrupted!
That means all cable grounds must go to the box. You are trying to wire them to the receps and ignore the box, and that's wrong.
So you must - always with metal boxes - you must get a #10-32 screw and put it in the pre-tapped hole made specifically for grounding. You then pigtail off of that.
"But that's now 7 wires I have to get on a nut!" Not quite. There are several ways for receptacles to take ground from the box. With switches, it's automatic!
Or, feel free to drill more holes and use self-tapping screws, as long as they are -32 thread pitch or finer. You can have as many ground screws as you want, just don't put them on knockouts!
In many cases the receptacles do not need a ground wire. They can pick up ground several ways:
- They are making hard, flush metal-metal contact with box parts (i.e. screw bottomed out, not floating on a drywall ear). Doesn't work if there's rust or paint or those little paper squares that capture the screw.
- Some receps have a scraper that touches the screw thread; these are called self-grounding and can pick up ground off the screw thread even if the screw isn't tightened down hard flush. You may find that feature on many GFCI receps.
- You can also have a pigtail from the recep to a ground clip that grabs the edge of the box.
- Switches can ground via the screw heads even if they're floating up on the drywall ears.
Use gray tape to distinguish the neutrals.
Here's the trick. In cables, white wires can be remarked to be hot, but that is the only remarking allowed (can't do black to neutral) until you get to #4 wire size. So...
On the second cable, enthusiastically mark the neutrals on both ends with gray tape (e.g. spiral down the whole wire). That is the allowed "alt" color for neutral. Also lightly mark the hots with gray tape (e.g. a wrap the width of the tape). This does not make black into a neutral, because it can't. This clearly identifies the pairs.
The metal boxes must have proper cable clamps
I thought I heard you say you were feeding all 4 cables through a knockout. I could see where four 12/2 Romex could be crammed through a 3/4" knockout, but not legally. Every Romex cable entering the box needs to do so at a listed cable clamp, whose purpose is to prevent the wire from taking damage from the sharp box edge.
Don't use LOAD unless you are meaning to protect onward outlets.Almost every GFCI (except the dangerous Chinese cheapies sold on Amazon) will support 2 wires under the Line screws via the "screw-and-clamp" back-wire technique.
Question 1: A 2-pole breaker is totally fine
Yeah, go for it. Obviously the common trip functionality is unneeded and might be annoying, but there's no harm at all in this.
Question 2: Daisy chaining wire nuts is fine.
Yes, you can handle large numbers of wires on a nut by "daisy chaining" 2 nuts with a pigtail in between. Remember the stuff I said up top about "Ground the box FIRST" so for instance have the box pigtail run through a Greenie where it picks up the 4 cable grounds, and then goes onward to a regular nut where it picks up the recep grounds (if those are even needed... if you have self-grounding receps or flush contact, don't even worry about that).