Operating System and Two Boot Drives

Hey Y'all,

I just bought a new 256 GB SSD external drive for my mac. I am going to put Final Cut 7/Old Adobe Suite on it as a separate bootable drive from my mac. Question, can I just copy OS X Lion over to it and have both my internal and new SSD using the same operating system, or do I have to buy another copy of OS X? I'll never be using these two partitions at the same time but wasn't sure about creating another boot drive. Can anybody tell me how to do this, and if I need to purchase another copy of Lion to do it?