You create the shared folders in the server console and there is an option to enable duplication, thats all there is to it. As long as you have more than 1 drive it the pool WHS will take care of making sure every file in that share is on 2 drives.
Its not identical to Raid as you really dont know where your data is on the server, which in most cases doesnt matter. You can access it via \\homeserver\whatever and you know its on 2 drives so you are covered in the event of a drive failure. It may be hard to give up that control, I was uneasy about it to start, but then I saw the benefits. No more worrying about having to replace an entire set to increase space or waste space because I bought a bigger drive than my existing array, etc. If I need more space I buy whatever hard drive I like (or is on sale) slap it into the server and add it to the pool. WHS will use it and shuffle around items as needed.
All that said, yes you need to backup your data if its important to you RAID = redundancy. Redundancy != backup. If your WHS already has 2 or more drives in the pool you can copy right from your raid array and it will be duplicated on the server as you move them over. If you only have the OS drive and the 2 raid arrays you either need to make a 3rd copy and add the drives to your server then copy from it or break both Raid sets use one drive from each array to make a pool on the WHS, then copy the data from the other 2 drives and add them when the data is on the pooled drives (this scenario also assumes that both of your Raid sets are not full).
My WHS is 2x2TB, 3x1TB, 2x640GB, 1x200GB. Pretty much all mixed brands, spindle speeds, models, and interfaces but they all play together in the pooled storage.