ISP’s switching

I have primary link from ISP A and for redundant purpose we are getting one more additional link from another ISP B. ISP A gives serial output and ISP B gives Ethernet output.

Can I implement the following?

1) I want to use both the links simultaneously
2) When ISP A fails ISP B should take over
3) When ISP B fails ISP A should take over

In this situation

1) How does the router decide to switch when there is a ISP?s failure

Is it when local loop is down or ISP backbone is down or slowness in the link?

Kindly advise the same?

Thanks,
Sridhar Kamath