It can be a tough call, but I would definitely go for a DCN if the price works for you. It will be plenty of room for 2 girls. It's easy to clean the cage or get rats out of the cage. You can close the rats into one half and clean the other half. If you ever consider geting more rats, you will have plenty of cage. More cage is definitely better and more fun for the animals. While all that room may not be necessary for just 2 girls, I would get and recommend the DCN if the price works for you.
I had the same dilemma with my two girls (now three). The general mantra I heard is that there's never such a thing as too much space for rats. So don't think that overkill is a factor. Pricing and space should be your main concerns.
Fortunately, space likely isn't much of an issue. A DCN doesn't take up more sq. ft. than a SCN. It takes up more vertical space, which could be an issue if you intend on putting the cage on a table or someplace with a low ceiling. I imagine that the DCN is not a problem for 99% of rat homes.
Price is probably your bigger concern. You could get the SCN now and buy an add-on later, but as pointed out that is more expensive. A SCN is already quite an investment, and you can probably justify the additional cost since you're already spending that much money anyway.
It did seem funny that I had so much space for only two babies, but they seem pretty happy in it. They can run full tilt from top to bottom. I removed two of the ramps so they spend most of their time climbing and getting exercise.
I agree that if you get the DCN, to keep them in one half first. I wanted to encourage my girls to poo on the bottom level, so I blocked the top part and let them get used to the bottom half for a week. It also meant I was scooping up poo from a smaller area, but that didn't last long. Either the litter tray I bought or the spaciousness of the cage helped them become litter-trained almost overnight. It was amazing. It was fun building up the top part of the cage while it was closed and then lower the ramp so they could explore. Good times.
I think if it were me I would get the dcn right from the start, since the addon is quite a bit by itself. If I was worried about space, I would probably not go with a critter nation at all for 2 rats, I would go with a flight cage or something, that gives a lot of space but not as much floor space, that I would not have to bend over to clean or see them easily.
If they are girls, they will use all of dcn anyways. Like someone said above, you can clean one side if you choose to and lock them into the other. If you end ever getting more you have the space already and if they don't get along you can keep two groups separated even in one cage.
I'd get the DCN and then use half at a time... As someone else suggested, close them into the top half (use the bottom pan in the top or buy a second top pan) then you can clean one side, move the rats into the clean side, and then clean the dirty side. I did this with my group for a while and it worked GREAT. Now, I am using both halves split into two cages (males on top, females on bottom) and it works great that way too.
You might want to wait a while to order though (unless that price is really good) because the DCN / CN seem to go on sale near Christmas.
Someone recommended a website to me when I was looking for my DCN: camelcamelcamel. You can tell it to alert you when the price for a DCN drops below a certain point. I know it searches on Amazon, but am not sure what other sites it searches on.