As you have no doubt discovered by now, backing up a dialer with another dialer is not as easy as it seems. The problem, of course, is that dialers are always up, so floating static routes are ineffective. In a controlled environment (where you control the routers at both ends), it is fairly easy to do with dialer watch and filtering of target routes (see the white paper on my web site). However, if you do not have the luxury of running a routing protocol, it gets amazingly difficult. You may be able to "ping based routing" but I am not sure how well that is supported on the 800 series. Worse, if you only want the primary link up when it is needed (dial on demand), there is no known IOS supported reliable way to achieve failover.
You're doubly hosed if the router must do a different NAT for each dialer. In that case, it is not enough to just start using the alternate dialer, you also need to clear out all the old NAT translations for the defunct dialer (hint: NAT policy maps only apply when setting up a NAT and are ignored once the NAT is in the translation table).
In the past, those with a need have addressed it using external black boxes to do the job, but be aware that ad hoc solutions have a nasty habit of only working most of the time, and failing when you can least afford the down time.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
