conditional formatting between sheets

Alright.. I'm making a table where I'll be using values from one sheet (sheet2!), to compile a list in another (!sheet1). I want to make sure I use every value in sheet two, but no more than once. I was trying to use conditional formatting to highlight the cell in sheet 2 after its used, and make the text bold and italic if it was used twice. Any ideas on how to do so? I can't figure out how to enter the formula manually. I realize it would be easier if it were all in the same sheet, but I'd like to keep them separate.

Also, just a quick thought/question.. If one refers to a library in excel, what exactly does that mean? Is there somewhere you can enter values to use? Or is a group of data (like mine in sheet2) just considered a library?

Thank you in advance, guys