If you have a piece of cardboard or something a little sturdy, you can make a divider out of it. Wire works best since that way you can insure the heat gets to both sides but if not, then put the older chick on the cooler side of the divider.
You could also take a second water and food container, put it and the day old under a colander (like for draining spaghetti) in the brooder. It works well, gives the little one some room to move around, prevents the older chick from making a snack out of the younger one, and still allows for both of them to get to know one another. You weren't gonna make spaghetti tonight anyway.