I think your requirements can be met.
You can create two dialers using only one B-channel and they will support either IP or IPX.
However, you do not need to have two dialer interfaces to the same destination out on the BRI interface since you can have the two interfaces bundled under one dialer interface and run both IP and IPX on it.
Here is a sample configuration that provides the functionality you requested:
int serial 0
ip address 10.2.2.1 255.255.255.252
ipx network FACE0FF
backup interface bri 0
backup delay 0 0
int bri 0
no ip address
encapsulation ppp
ppp authentication chap
dialer pool-member 1 max-link 1
dialer pool-member 2 max-link 1
int dialer 0
ip address 10.3.3.1 255.255.255.252
dialer-group 1
dialer pool 1
encapsulation ppp
ppp authentication chap
dialer remote-name ww-tac
dialer string 222222222
ip ospf demand-circuit
interface dialer 1
ipx network BABE
dialer-group 2
dialer pool 2
encapsulation ppp
ppp authentication chap
dialer string 222222222
dialer remote-name ww-tac
ipx watchdog-spoof
ipx spx-spoof
dialer-list 1 protocol ip permit
dialer-list 2 protocol ipx permit
In this configuration, the backup is done on the physical ISDN BRI interface. If the WAN inteface is active, the backup remains down. If the WAN interface goes down, the backup comes out of standby and the dialer interface dials out to the remote router, setting up two connections: one through dialer 0 for IP and one through
dialer 1 for IPX.