I have seen threads that discussed this before but do not know where to find them. If you are talking about having different people in the second room rather than the first then you just book as you would regularly do -- make one reservation and then make the other.
If what you want is two reservations for the same people but want both for a while because you are not certain which one you actually want to keep then there is apparently some risk that MS would cancel one of them. Or, at least,some have mentioned that has happened, but can be avoided as long as you simply use a different lead name on the reservation (if husband and wife are both members reserve one room with the wife as lead and the other as husband). In fact, you can just put a lead name on the reservation and wait until later to add any others to the reservation you choose to keep.
As to personal experience, I have actually made two reservations for the same time at the same resort on line, both in my name and added other names later (I do that when relatives will be using the second room but I do not know yet which ones are actually coming). I am guessing the cancellation risk exists, if one actually exists, only if you actually contact MS about the reservations.