Routers / Switches :: How To Connect Two Internet Routers To One LAN

How do i connect two internet routers to one IP address range?I currently have one broadband router connected on 172.16.0.3 subnet 255.255.0.0, I have a second line which is a copper ethernet EFM 2Mb line connected using a Cisco 1800 router configured on 172.16.0.254 subnet 255.255.255.0, no DHCP is configured on either router. the dhcp is provided by a windows 2003 server on our LAN which is also configured on the same LAN IP range. if i connect the Cisco router into the same switch as the broadband router my network devices stop working and dont respond.

If i check the broadband router it shows all the connections to it as having the same MAC address as the Cisco router?? if i disconnect the Cisco router everything returns back to normal and works perfectly.What I want to achieve is to be able to run both routers on the same LAN to provide internet access by configuring the pc's to route through a specific router. the DHCP I want to stay routed through my original broadband line. at the same time i want all pc's to access to the lan devices (ie. servers and printers) what the correct configuration would be for the two internet routers would be?