I seem to remember reading in a very early article that the other lacrosse team house was just around the corner from 610 N. Buchanan, just a couple houses away.
To my mind, the most likely reason for the DA to seek to seal the other search warrant documents is because making them public would jeopardize an ongoing police investigation, in a way that allowing the other search warrants to become public would not.
Here's some speculation: Perhaps the police found drugs at the second house. Perhaps even Rohypnol or some other date-rape drug. That may be why Nifong is "hinting" to Newsweek about a date-rape drug being involved in this case.
The DA/police may be trying to lean on the person in whose room or possession the drugs were found, in order to find out where the drugs originated.
Or it could be drugs were found, maybe cocaine or a large amount of cannibis, and the police have launched a separate ongoing investigation, unrelated to the rape allegations, as to drug possession and/or dealing.
You're correct. There is a second house on Urban Street which is/was occupied by lacrosse players.