but the screws that go through the faceplate aren't passing through a threaded hole on the outlets but through a slot, so they need some sort of nut on the back. I've never seen packets of tiny hex nuts that would fit those screws. Is that something you can buy?
Those faceplates are made to use 8-32 bolts (machine screws) with nuts - so 8-32 nuts. They are usually provided with the domed cover and are usually the kind with an integral lock washer. You have two choices to acquire those.
- Your local big-box store sells 8-32 bolts (the shortest ones should suffice) and 8-32 nuts in packages for $2-5. Some packages have matched sets. (It's difficult to charge less than $1.50 due to the production costs of sealing a thing in a bag with a barcode. To make that seem like less of a ripoff, they give you many).
- Your friendly neighborhood family-owned hardware store, that has been there for 100 years that you will never shop at because of some nonsense about prices, has a whole wall full of slide-out drawers full of every size of hardware. They will cheerfully sell you short 8-32 bolts and nuts for a nickel each. Choose a head style you like, you won't find a better selection this side of McMaster-Carr. The price of each thing is marked on the lid. They provide little bags, you are on the honor system to write down the price of each item and the quantity.
If you already have 8-32 machine screws, sorry bolts, and just want nuts for them, take one into the store and test fit nuts from the bins onto your bolt. People do this all the time, no one will accuse you of shoplifting. Just make sure you count the nut that you left on the bolt! When test-fitting never force it - it should roll free if the bolt is undamaged.
I am replacing a single 220V receptacle with two 20A 120V GFCI receptacles (USA). The white neutral wire was capped off in the receptacle box; I've made sure that it's attached to the neutral bar in the subpanel. I'm going to connect that white wire to two white wires, one for each outlet, using WAGO-221 clips.
I gather you've already figured out that you need to pigtail the wires to the GFCIs before you install them in the domed cover. Pigtailing neutral is mandatory anyway due to this being a MWBC (Multi-Wire Branch Circuit aka shared neutral).
Fitting two GFCIs abreast in a 4" steel box is ambitious. I find it very difficult to do, myself - in particular side entrances of cable clamps or conduits will foul the GFCI side terminals and pin the wires in a way which would damage them. That's why I use 4-11/16" boxes and domed covers for 2-gang GFCI (or 2-gang with a GFCI in either position). So, coping techniques: #1 change to 4-11/16 box. #2 rotate domed cover 90 degrees if it'll keep you away from wire entries. #3 stack on a 4x4 "extension box" so you have a full 1.5" of clear space behind the domed cover, and another 1.5" of just wire space.