I don’t understand what you mean here. You can open multiple instance of LightBurn, but only one can talk to a given laser at one time.
For the library, the thickness values will be displayed on inches or mm, depending on your measurement settings, and that is used to choose the material thickness that your setting is for. I don’t cut 1/4” ply with the same power I use for 1/8” ply. The only thing displayed in the library itself is thickness - what other measures were you expecting to see in inches?