You guess correctly.
SKY+ only has a single video stage linked to a single decoder, very short sighted of SKY (from a viewers perspective).
Of course this helps to promote their multiroom offer
I am assuming you don't mean record SKY onto video the watch the video in the other room while someone watched SKY in the other, have to ask
If you don't mind paying an additional £10 a month then the multiroom deal is pretty good (phone requirments taken into account) but it depends a lot on how much subscription telly you watch.
It could be better value to have Freeview or a FTV Dsat system in the other room with SKY+ as your main source being TVlinked to the second location.