If you are using 2 ISP's and want to route the same IP you must have your own IP space from ARIN or APNIC if you are not in the US. The ISP will not route any traffic for you that is less than a /24, so you need to make sure you have a block size that large if you are going to be multi-homed.
With using 2 ISP's BGP will be the routing protocol used. You will also need your own ASN.
It is possible to have 2 ISP's and use different subnets. This can be used to help split the traffic load, you can direct some users out one connection via NAT overload and other users out another connection.
This is a bit harder to manage but also doable.