You have to take a few things into account. Teams like VT, UVA, and MD do not want a N-S split. You want to keep the divisions competitively balanced. You want to keep as many rivalries together as possible. Also IMO it would be easier to have the divisions remain for BBall as well. Taking all of these into account the best split would be...
This should keep everyone happy. You don't lose any major FB rivalries. You give VT southern teams. You keep the ACC core 4 together for both FB and BBall. NC St and Wake would play UNC and Duke only once a year in BBall, but someone has to suffer. Also FSU and Miami would only play once a year in BBall. But they have only been doing that for a few years anyway, so who cares.
For FB you play your division mates every year. You play your cross over rival every year. Then you play 2 of the other 6 on a rotating basis. So you play those teams once every three years. Total FB conf games 9.
For BBall you play 19 conf games. Divisions teams you play twice a year. Cross division you play once a year. I hate the idea of not playing everyone in your conference at least once a year in BBall. Also I hate not knowing who you play twice a year in BBall. Being in the BE it was never the same. Keeping the divisions for BBall keeps everything nice and clean. For SU and Pitt this is only one more game a year in conf. However for the ACC teams it is 3 more games. Hopefully that isn't an issue. You can play 9 home games, 9 road games, and 1 neutral site game. The neutral site games will be an ACC double header in a market that the ACC covers but needs more exposure in. These would be Boston, New York, Philly, DC, Orlando, and Tampa. So the games would be as follows...
North- a double header in the Boston Garden. One game featuring BC, the other Duke.
Mid Atlantic- a double header in the Wells Fargo Center. One game featuring Pitt, the other MD.
South- a double header in the O-rena. One game featuring FSU, the other Clemson.
ACC home- a single game in the Greensboro Coliseum featuring NC St.
North- a double header at MSG. One game featuring SU, the other UNC.
Mid Atlantic- a double header in the Verizon Center. One game featuring UVA, the other VT.
South- a double header in the St Pete Forum. One game featuring Miami, the other GA Tech.
ACC home- a single game in the Greensboro Coliseum featuring Wake.
Having these neutral site games will increase the ACC's presence in each of these markets and make the ACC brand more valuable. Plus it would cause a buzz both in those cities as well as nationally. ESPN would love to televise these and hype them up.
Anyway that is what I would do if I ran the ACC.