All you need to do is connect the single cable to input 1 and set a manual recording on a channel you never watch lasting 24 hours repeated daily which will always fail but take virtually zero hard disk space. Delete these failed recordings when the next dummy starts.
This works because input 1 is the channel normally used for viewing (from switch on with nothing recording) and input 2 is normally used for the first scheduled recording. If an input is already in use the box will automatically switch to the other input for whatever it wants to do. Because the dummy manual recording is already telling the box that it is already using input 2, anything else will happen on input 1. With this setup you can do any one of three things
1 Watch one channel.
2 Record one channel (either timed via Planner or by pressing Record whilst viewing).
3 Record one channel and watch a previous recording.
For recordings from the planner it is best to leave the box in standby and doesn't need to be left on any particular channel (DO NOT leave on channel dummy recording is set on)