Connect two routers via ethernet ports, use same ssid, password and security type, but use different channels. Disable routing and dhcp on the second router (the one not connected to wan). Assign static ip, different from ip on router 1 to router 2 for management purposes. After that wifi roaming should work. Sometimes consumer class routers don't like this kind of setup. If you experience errors, use different ssid's on routers and select between networks manually, depending on your location in house.