Dont know if u can use default routes with different AD (because default route being the route of last resort, it may not have any AD associated). However, it is very much possible with route-map.
For e.g. lets say site A. Create a route map, match any traffic coming to this interface for internet, set interface se y/z (this is interface connecting site A to ISP providing internet). Also, set another interface in same statement. This interface is the 1 pointing to B.
In route map set command, u can specify multiple interfaces, where 1st interface acts as primary & 2nd defined interface acts as backup whn pri int is down.