As far as I know you can only do this on a 7500 series router, I may be wrong though, it's known as the Multilink PPP inverse multilpexor function, you need the following:
Create the multilink bundle:
interface multilink <group-number>
ip address xxx.xxx.xxx.xxx yyy.yyy.yyy.yyy
encapsulation ppp
ppp multilink
Assign an interface to the multilink bundle, in interface config mode (whichever interface you want to be part of the multilink bundle):
interafce xxxxxxx
no ip address
encapsulation ppp
multilink-group <group-number>
ppp multilink