A Lightroom catalog is simply a database, but one that contains only metadata, not photos. Lightroom doesn't care where your photos are nor how they are arranged on the harddrives. So having two catalogs provides no separation between JPEG and RAW, since the images are not in the catalog. To my mind, having two catalogs will only create issues, and get in the way of managing and processing your images, as you will have to open and close catalogs and 'remember' which you are in.

One workflow altering change that LR brings to photoproccessing is that JPEGs are completely disposable. There is no need to keep them. Since LR does not alter the RAW file, and keeps processing "recipe" history, you can create a JPEG, identical to the last JPEG, anytime you like. You can create unlimited virtual images, each with a slightly different configuration, and create different JPEGs on a whim. I found this both frightening and ultimately liberating. As a result, I no longer save or catalog JPEG, as they are redundant. I create JPEGs to put in iPhoto for family viewing, or on harddrive for sharing or posting on Smugmug, but I don't save them in LR, since they are no different than the RAW images + Processing.

Rather than reimporting JPEG back into LR, which is easy to do, I recommend you leverage LR 4's Publish facility. Rather than Export, you Publish images to your NAS. Of course, technically, this creates JPEG (or whatever you choose) and saves it to the chosen directory on the NAS, but it does not create a separate image record in your LR catalog. At any time, you can see a view of what has been Published to this NAS from LR, which essentially shows all the JPEGs, arranged in folders, etc. This does not store the image in LR, nor does it re-catalog the image...it is simply a record of where the image was Published. Critically, any change to the image: tags, keywords or even processing, triggers the corresponding image to be Re-Published, simply awaiting your approval. This means if you change a tag or keyword, you don't have to re-export to the appropriate directory or even remember where it goes...you simply Republish, LR does the remembering for you.

In the Library Module, scroll down on the left hand side to Publish Services. Click on Hard Drive, and follow the setup dialog box, then drag a few images to the Publish location. Give it a whirl.