It shouldn't be a big deal, but you would have to first remove the old vanity and then open up the wall and do some re-plumbing. You would cap off one drain and one set of hot and cold pipes in the wall. Then you would need to route the other drain and set of hot and cold pipes laterally within the stud space so they would be in the center; this might require notching out one stud, depending on where things are located. Which set of pipes you would choose to cap off might depend on where things were in the wall in relation to the studs: it's possible that one side might be easier to re-route to the center than the other, or one side might be a shorter run to the drain. It's hard to say before you open the wall, but that's the general idea. Once you re-did the plumbing and repaired the sheet rock, you would be ready to install the new vanity. Oh, and if you don't have shutoff valves under the sinks, use this as an opportunity to install a set on the pair of supply lines you will be using.

Hope that helps. Feel free to re-post if any of this is vague, or once the wall is open.

I would cap off one side and keep the other sink on the end so that you can have a wider set of drawers and continuous countertop. If the vanity abuts a wall, put your drawers on the wall side.

All the replumbing can be done without accessing the wall.

Joseph-

I agree that that is possible. However, I also took into account that, if they were buying a stock vanity, the back and/or the drawer arrangement might interfere with drain and water supply lines if they were brought laterally across the outside of the wall.