Are you planning to keep the ports on the same face as the single port would be? You're going to be adding more width because of the extra port wall needed to make two ports versus the one. Craig is right though; each port would be 2.5" in width and each port would have the same length as the original port length. I don't really like having tall skinny ports, especially if you're going to be running a lot of power through the subs. A single larger port would be better.
edit: about the end correction - it will depend on how the ports are positioned within the enclosure. I wouldn't take any end correction into consideration, as it would only be an inch or two different with two narrow ports like that. If anything always err on the smaller side rather than the larger side when it comes to port, as the cabin usually couples and loads the ports and reduces effective tuning anyway.