lammert - 1:57 am on Jul 7, 2010 (gmt 0)
Hi dsjbirch, first of all Welcome to WebmasterWorld!
I assume that your server is answering requests from global visitors on the DSL lines. Both DSL lines will have unique IP addresses for the outside world, so the first step is to have two A records in the globally visible DNS configuration for your domain name where each of the A records contain a public IP address of one of the DSL routers. Whenever a visitor requests the IP address for your website, it will be assigned at random one of the two IP addresses in your DNS setup. I.e. about 50% of your visitors will come in via DSL line 1, and 50% via DSL line 2.
When one of the DSL connection fails, 50% of the visitors will have a problem because they have been assigned the IP address of the connection which is down. That can be solved by using a DNS service provider which automatically temporarily removes A records from their round-robin scheme when the IP address associated with that A record becomes unavailable.
Because home Internet connections are not industry grade with a high guaranteed uptime, it is often better to move your server to a data center with higher quality Internet connections if the server gets significant traffic and generates significant income.