Hi,
And welcome to the forum
The simple explanation is that you will not be able to do this unless you have another ariel or if you can split the ntl signal. Splitting the ntl signal is easy enough with the right splitters but use the type you would for normal tv and you could ruin the signal your stb/cable modem would be getting.
If you can tell me how many scart connections each one has i'll try do you a piccy of how you can best fit it together.
mark