Oh, you poor things! It's so hard when there is only one. Even though the other one will be a week older if it is just the two of them I think they'll be just fine. They'll be glad to have a friend (especially your baby).
Sometimes with incubators there will be quite a big temperature difference inside because they aren't insulated (like the super expensive, top of the line ones are). If you wrap a towel or blanket around the incubator, leaving the vents uncovered, it should help to stabilise the temperature inside. I had to do that last time. I tried adding 'heat sinks' (I used bottles filled with water) but it wasn't until I wrapped a blanket around it that there stopped being such a big temperature difference, even though ours has a fan on it.