Fail-over steps for Internet Explorer

I am looking to find a way to setup multiple proxy server addresses within Internet Explorer so if the local internet access is down, the browser will attempt to use the next proxy server listed so users can access the internet from that location. We have 3 offices with 2 offices having an internet connection using ISA Server. We are setting up a disaster recovery process where if an internet connection is lost in one office, the users there will be automatically directed to the other office so they can get to the internet without the end users making any changes. Can this be done?