If a conference wants both of them there is no reason they can't take both. There are advantages to it. I just think there are more advantages (on the money side of things) by taking only one (preferably UNC).
I think if the SEC came along and wanted more markets, it would be better to take UNC and a Virginia school (I'd personally lean toward Va-Tech over UVA at this time) because all three of those schools can carry their state if they are playing top level football. A move like that would seal the ACC's fate as a lower level conference. At that moment both state government's would be stupid to not have a team playing top level college football just so their schools can stay together.
I just don't think you need both UNC and NCSU to carry North Carolina. I think just UNC and you own that market. Possibly not in basketball but football you would. By taking only one you could gain another market with another team rather than doubling up.
I think that's what Mike Slive's ultimate goal is - to bring in UNC and Virgina Tech. He'll probably settle for NCSU and VT but wants
UNC. Even then, N.C. State would be attractive to the Big 12 and would fit in with Georgia Tech, Miami (if both wanted to go to the Big12), Clemson, FSU, and probably Louisville.
The Big Ten could choose to take Duke too if they wanted to try to get an in-road into the N.C. market and two of UVA, Maryland, or BC to pair with Notre Dame (likely UVA and BC, imo).
I believe North Carolina, Virginia, Maryland, and Georgia Tech will not leave the ACC so long as there is a possibility of being invited to the Big Ten. And because the Big Ten does not appear to be in any hurry to expand, don't expect any of these schools to move anytime soon.
If they want to expand to 16, the SEC should pursue Virginia Tech and NC State, for two reasons:
1. They are arguably the two best football programs of these 6 schools
2. They are not being blocked by the backroom agreement between the universities of Florida, Georgia, South Carolina, and Kentucky that prevents adding schools from those states
If the Big 12 wants to expand, they should target schools being blocked from the SEC, such as Florida State, Clemson, and Louisville.