depends how far away your main socket is?
it would probably be better to go from the main socket but i see no reason why it shouldnt work taking a third one from the second. (sounds a bit wierd lol)
if it is working with an extension from the second socket now then just put a doubler on that socket and run your wire from there.
if you wanted and it made sense, you could run the wire from your primary socket to the broadband connection then take it from there to the other bedroom (making that the third socket)
trial and error seem to be the answer along with what is practical.