I was thinking more along the lines of having a separate SSID for the secondary ISP. If my wife needs to work, and primary ISP is down, all she has to do is connect to the other SSID.This is probably best addressed with a spare router if you have any kind of authentication or manual configuration required (primarily name servers). This is perhaps the most high-maintenance approach from an end-user standpoint. If both services support DHCP (perhaps becoming more common), you wouldn't have to do anything other than switching which modem is connected to the WAN port.
If you're using a gateway on one side or the other, you're probably going to regret having got out of bed that day.