Are you trying to simultaneously use two ADSL lines coming into your house? If you have a PC with two network interfaces, it will be simpler if you do not put both routers on the same network. Connect the PC to both routers and enable load balancing in the Network Control Panel on Windows.
If your computer only has one network interface, then it's quite a bit harder. You can connect the routers together with an Ethernet cable. Configure both routers to use the same subnet. You would also need to disable the DHCP server on one of them. That's the easy part.
Now, you have 3 options:
1) You can configure two default gateways in the IPv4 settings in the Network Settings. The computer will still only use one gateway but will use the other one if the first one fails. The failure detection could take up to a couple of minutes, so that may not make for a smooth, seamless experience.
2) You can configure static routes to each gateway on the computer. Basically, you computer is using each router to reach separate parts of the Internet. This is hardcore, manual configuration.
3) You can enable the RIP routing protocol on the routers and the computer to exchange routing information. Then you can configure the routers to advertise just the default route or, optionally, more-specific routes to the Internet. This is a more dynamic way of doing either option 1 or 2. This is still pretty hardcore.
Microsoft has an article for doing options 2 and 3.
windows.microsoft.com/en-us/windows/configuring-multiple-network-gateways#1TC=windows-7