I had never heard of the Archos 404 until I read your question but I've just been reading about it on the Archos website, so I think that I can answer your question. (Unfortunately, you won't like the answer).
Whenever you're watching or recording TV, every 'active' channel (i.e. any channel being watched or recorded) requires its own separate tuner. For example, if you've got a basic TV with an old-fashioned VHS video-recorder, you can watch one channel on the TV and record another onto VHS tape because there are two separate tuner circuits in use. (The tuner inside the TV handles the programme you're watching, while the one inside the video recorder deals with the programme you're recording).
As far as I can see, from the Archos website, the Archos 404 does not have a TV tuner built into it. It uses the tuner inside the TV you're watching to provide the signal which it records. If you've only got one TV tuner circuit in use, you can only have one 'active' channel. So it won't be possible to record a different channel to the one you're watching.
Sorry!
Chris