Why can't I both record a channel and watch another on sky plus?

I can't say anything for sky plus, because frankly I've never heard of it, but if it's not a set-top box, chances are you can't get more than one channel streaming at a time. For instance, when I'm recording TV shows on my laptop, I can't watch any channel but the one that's recording. Likewise, picture-in-picture is not available, simply because my tuner only supports one stream at a time, whereas a set-top box allows you to view more than one stream at a time, offering simultaneous recording and watching as well as picture-in-picture. Hope this helps!