I'm still not very clear but it sounds like you have two lines into your site which you cannot rely on. Are you suggesting saying to your users, "if you can't get Moodle here, xxxx, try this instead, yyyy". I'm sure where to even start with that
One thing that isn't is a "simple backup"! I don't know what the technology is behind your connections but lines don't have IP addresses. If nothing else, you could start by turning down your DNS TTL to 5 minutes which would at least reduce your down time. You could also use a "round robin" style arrangement. In an ideal world you would be able to park something like haproxy on the remote side of your connections. Properly set up, TCP/IP was specifically designed to work around problems like this. As I say, not remotely simple.