There are a number of the asus routers that now support dual wan interface. I know things like the ac68u can do it but many of the other ones also if you update to the latest firmware. The tricky part is what is "down". Seldom does the equipment power off so you will still likely need to tell the router to use the other connection.
You can also do this directly with your PC but it is messy. You would put the second ISP on the same subnet and give it a address such as 192.168.1.2 assuming your main router is 192.168.1.1. You would need to disable the dhcp in the second router.
When you want to switch you would issue a route delete command for the 0.0.0.0 route and then a route add command for 0.0.0.0 telling it to go to 192.168.1.2 now. This is all done from the cms window on the PC. You would have to do it on every device so if you have a lot it will be better to get a dual wan router.