I remember your earlier post - can't exactly remember what advice you had about it.
If you are putting a radiator into a conservatory and want to take the feed off a radiator on an adjacent external wall to the new conservatory (by drilling through the external wall to be able to run pipework into the new room) then I can confirm that I have successfully done this.
The point is that the 'old' room which now has a conservatory on the outside of it will now need less heat supplied to it.
One merely takes a feed and return, one from each side of the old rad. I found I was able to completly turn the old rad off (though left it in situ).