Advantages, and disadvantages for keeping 2 pet mice?
Mice are social creatures and female mice really prefer to have a cagemate. This is important for their well-being because it's been proven that mice with a cagemate tend to be healthier and longer lived than if they are forced to live in solitary confinement. Mice who have a buddy tend to be calmer and easier to handle and train.
Most male mice, especially American strains, are very territorial. So if you choose to keep two male mice together there is always the chance that they will start fighting and you will have to separate them into their own cages. Having two mice means that their cage will have to be slightly bigger than a cage for one mouse (a ten gallon tank is perfect for two) and it's a good idea to provide them with an extra wheel. It's not a lot more expensive, but two more mice will obviously eat more food than one.
Mice are not very vocal, but they do run around in their cage and chew a lot, especially at night. If your cage is made from plastic then yes, this can get pretty loud and annoying.
Mice do have a smell, but it's not as bad as you would think. Check out this other asker's question about mouse odor. I got best answer:
One last note, beware of pet "starter kits". A lot of the time these cages are actually way too small so the pet shop is trying to sucker people into buying them by offering a discount or a free pet to go with it. Before you spend money on it, check out this website to make sure that the cage and supplies that come in this kit are actually good for mice. I wouldn't want to see you get ripped off!