Network issue with DHCP

The office I work in has 2 servers (one for each organisation in the office, and on different IP ranges - 192.168.168.x, subnet 255.255.0.0, and 192.168.167.0, subnet 255.255.0.0), we share a number of printers that have static IP address (192.168.168.x, on subnet 255.255.0.0.). Each organisations Laptops and Desktops access the network via their own Airport Extreme wireless network. DHCP is handled by each XServe. Due to the desire to share printers, both networks are joined through a network hub. I have a problem in that I have no control over where each machine gets it's IP & DNS info from. Does anyone have a suggestion, other than just not sharing printers, on how I can control the DHCP allocation to the correct machines? Is this possible? Machines print directly to the printers and print sharing is not turned on on the servers. I hope someone can help as it is driving me made, as it interferes with my network clients logging into the network & their files. Yours Justyn