I suggest you go to www.cisco.com and search for ConfigMaker. It is a free download, and can help greatly with these tasks.
The below approach causes one circuit to kick in, and then a second should the amount of traffic require it (probably saving you some call charges).
The important bits for your config are:
interface Dialer 1
ppp multilink
dialer load-threshold 10
dialer-group 1
interface BRI 0
dialer rotary-group 1