There is a manual fix you can use which will allow you to use the sky plus to tape with only one feed, although it obviously will not allow you to watch two channels at once. You will however be able to record something and also watch anything else you have recorded at the same time.
If you want to go ahead, then go to live tv and record whatever happens to be on.
Then go back to the sky menu and choose manual recordings. Set up a daily recurring manual recording for 23 hours and 59 mins, starting while the program you are recording is running for (eg. you have gone to live tv and recorded whatever was on at 9.35 AM until 10.00 AM, you then set the manual recording to start at 9.40 AM and to finish at 9.39 AM). Also ensure that the channel you choose for the manual recording is one that you will not ever watch.
This process forces the sky box to manually record from the input that has no feed. Any future sky plus recordings that you then set will use the feed that does work.
It's not an elegant solution by any means, but it's handy if you only have one feed, or your second feed has stopped working (like mine did).