one domain Split

At the current time I have a AD domain 4 servers, 3 switches and two floors 5th and 8th. 2 of the switches link the 5th and 8th together via Fiber using Cisco 2924-XL. The two floors are wanting to be seperated into seperate domain but still need to share Internet connection and one other for special program via the 5th floor. Any help would be great.