Like in this image, there is 1 screw and hole for each coil, so you can put one coil in, and then put the other coil in. This image shows an RDA with one positive screw and hole, and both coils are screwed in at the same time.

When you put in a coil, you'll have it around a drill bit or screwdriver to keep the coil from getting out of shape when screwing it in, so it's a bit trickier to try to screw in 2, since you've gotta hold both of the drill bits or screwdrivers that are around the coils and then use your free hand to screw them both in.

The 2 positive posts just make it a lot more simple - hold one coil, screw in one coil, hold other coil, screw in other coil!

For wire, the gauge is wire thickness - lower numbers are thicker (like 24ga) and higher numbers are thinner (like 30ga). The thicker the wire, the more it wants to hold its shape, and vice versa. The thicker the wire, the lower the resistance per wrap, as well as more surface area.

In terms of building coils of different gauges, lower gauge will be more stiff and want to stay put, and higher gauge will be springier.

So you make a 26ga coil and get that last wrap in and put the legs where you want them, the legs will stay put more than 28 or 30 gauge. 30 gauge, it'll unwrap itself by half a wrap like a spring, and you'll have to bend that last wrap in and then some, so when it unwinds again, the legs will be where you want them to be.

It might be confusing right now, but when you try making something with 28ga or 30ga after starting out on 26ga, you'll know exactly what I mean when I say "springy vs stiff", and you'll know why I recommended 26ga off the bat :)