Are you using the second wireless AP as a bridge, meaning your Internet connection comes in on the other one, and then you're using the second one to get ethernet to your mining computer (it's bridging the two points with WiFi)? If that's the case, then you're really not doing ethernet, you might as well just run WiFi on the computer mining and be done with it.
Personally I would replace whatever is taking down your ethernet - if WiFi is more reliable than ethernet, something is seriously wrong with your setup and you're better off diagnosing that then trying to do some sort of failover to another connection.
No that's not what's happening, there are two WiFi hubs that are connected to phone lines via ADSL, and I happen to have a mining rig connected to one hub, but I was thinking how it would be possible to get mining rig to connect to the WiFi downstairs whenever some issue comes up with the top router/hub.
There's no specific reason why the ethernet is being taken down, I've just noticed that sometimes the router just needs a reset, and I can do it via 192.168.1.1, so if ever I'm away, I could still connect to my mining rig and then since it's ethernet cable is connected to the top hub, I'll just reset it from there remotely even though I'm not home. But if both are down, then bad luck obviously.