If i program a second spare remote, will I lose the functions in the first remote? Do I have to reprogram both remotes at ones? Even the one that already works?

I ask because I saw this video, he clearly states that this is the case:

 

All remotes have to be programmed at the same time.

 

Regarding programming remotes. I just programmed mine on my 1997 XK8 and found that it would only program with the hood, trunk and doors closed and windows up.