I'd like to see that happen to, but as far as I know, windows can only use one at a time even though 2 connections are available. As far as how fast the switching from one to another happens if one fails, it's hard to say, sometimes it happens seamlessly, sometimes you will have no network for about 30-60 seconds.
It's expensive. You can get routers (mostly enterprise routers) that have multiple wan ports. You can then set up a default route and a secondary router for outbound traffic and load balance the incoming traffic or even tell the secondary wan to only come online when the primary wan is down. Anyhow, be prepared to spend a lot of money for this. I haven't worked with enterprise routers since 1996 or so, but at the time they easily cost around $5,000.