I’m new to this stuff, so the method I use isn’t near as proper as that noted by Count Frolic.
I unwrap each object to their own UV as normal but I use the grid background as reference and then scale, rotate and position the individual islands so they only take up space in certain areas. I export the UV layouts and put them in a photo editing program as layers and combine them there. Then I use that layout to make my texture image. Back in blender if you use UV coordinates then each object can use the same material or texture image and they will all have the proper image applied.
This will work for diffuse and spec mapping, probably not for normal mapping though.
I will definitely give CountFrolics method a try though. My method is just trying to figure out things on my own with little foreknowledge.