They can be hardcoded to both use ppp0 or something. As long as they use different ppp network sockets they should work, but maybe that isn\'t possible by using Sharp wizards... You have to setup different routes, and remember that they both must have a default route, so one erases the other...
When pppd runs he brings a ppp socket up, and if ppp0 is already taken, it should start a ppp1, I never used dial-up, just used to setup a ppp connection with my laptop before I had the travel sync cable (the ir ppp connection is pretty slow compared to it, and I can have the Z by my side, instead of behind the laptop, I know about the tin foil kung-foo I could\'ve done, though... ), so I don\'t (and can\'t) try to repeat what you\'re doing to find out how you could do it.
Good luck!