HOw to setup two routers from two different ISPs to share same network

One of my clients recently subscribed to a second ISP and is connected using a UMAX router (WIMAX). The first Internet service is connected via an ADSL modem. all thier printers (2) are connected on the network through the ADSL router. when the ADSL provider is offline then the client switches to the UMAX provider and subsequently looses printer connectivity. Both routers act as DHCP servers and run a separate IP range:
ADSL router (HUWAEI): DHCP on, 192.168.1.1 -100
WIMAXrouter (Alvarion): DHCP on, 192.168.0.1-254

is there a way i can setup the two routers to be on one network and the client doesnt need to choose which provider to switch to (automatically done?) and never loose printer connectivity.

CFM, Zimbabwe