Having a second child can be exhilarating; everything you learned the first time around you can now bring to bear on the second child, and possibly have more fun this time.
But another child means having to find space somewhere. During the first year, you might get away with keeping the crib in the bedroom; you know you'll be spending enough time getting up to attend the newborn anyway, and this way you'll save some steps.
But newborns grow into babies, and then into kids. What do you do if you're not ready or able to move into a larger place, and the second bedroom is already occupied by the number one son or daughter? Double up, of course. The bunk bed is one of the greatest space-saving inventions. The function of the child's room shifts when a second kid is brought in: now, in the same amount of space, you've got to serve the needs of two kids.
This means not only two beds, but also two dressers and possibly two night tables. Remember that the kids will each want his or her own of whatever-it-is: a bed, a reading lamp, a piece of cake. To cut down on the sibling spats, make sure when you're designing the room that each child has the same set of things. They don't have to be fancy; the kids won't notice if they're using the cast-off reading plastic-framed mirror you've kept boxed in the basement.
To maximize function, consider having two small dressers rather than one large one. This will cut down on confusion when you're sorting through the laundry, and will emphasize the kids each having his or her own set of things.
Next, consider the mood of the room. Any child's room should have a playful, childlike quality, and this is especially true with two kids. A bunk bed helps with this mood: it serves not only as a place to sleep, but also as the frame for a fort or a gymnasium from which to hang over. In a shared room you do want to make it as easy as possible for the kids to keep the room organized, so a couple of brightly-colored toy bins and laundry hampers are in order. Again, these don't have to be fancy; oversized plastic boxes do just fine as toy chests, and have the advantage of being available in different colors.
Finally, you want everything to harmonize in the room as much as possible; a tall order with two individuals trying to exert their identities as they form them and then grow and evolve. Try to get the kids to agree on a color scheme for the room.. The colors don't have to be identical, but should be in the same range, such as blue and green, or red and orange.