Restricting DHCP Server

We have at our place two networks one of our company and second of another company the ip address range being 192.168.0.0 to 192.168.0.255 of our network and 192.168.1.0 to 192.168.1.255 of the other companies network subnet mask being 255.255.255.0we have now shared our leased line both the networks have a dhcp server and so now with both the networks connected. We face the problem that some of our clients receive dynamic ips from their dhcp server and vice versa. how can i stop this?