If a DVC Member who has reserved through the DVC site, you can put in a request with MS, via phone, chat or email (I usually do email as it assures I can easily make a copy of what I actually provided to MS and when, which is responded to usually in about a week), that provides the confirmation numbers of both reservations and lead names, informs MS that the reservations involve people who are "traveling with" each other, and requests to have the rooms near each other. If you have reservations for staying part of the time at one resort, and the rest at another, you need to make a request that clearly sets that out, including the different reservation numbers (if via email, I would likely do that via two separate requests)
You can make the request as early as the day you make the reservations, and likely should make the request no later than about a month before arrival to assure it is given before the resort starts assigning rooms to persons with the same arrival date. In the past when doing traveling with requests I have usually waited to put in the request until about four months out, after it becomes fairly certain that everybody on the reservations is actually coming.