Hello Vlaho,
Edit:
you can use a single IP subnet per PRI and so you can use a single dialer on the core router.
I would use two dialer interfaces on each remote with dialer map command
two floating default routes with one with lower AD
the suggestion is to use the dialer map command that allows to associate remote hostname, remote ip address, remote isdn number in a single command.
This helps also for authentication
you need then a username/pwd pair for each remote
about routing in this way you can configure floating static routes that uses directly the ip next hop of remote sites.
You then need to redistribute the floating static routes in your IGP
Verify both the failure and the restore events: the backup routes have to be removed from IGP when primary link comes back
Edit:
you can build a hierarchy of backup routes by using different seed metrics in redistributing floating static routes on the two core routers
Hope to help
Giuseppe
