Situation above ground forces:
You have two modems, or two ethernet connections, and want part of your connections to go through ppp0, and the rest through ppp1
Quick Fix:
Quite easy, get both your connections live, and ppp0 will by default be the default gateway. Now, use a proxy for all other connections, so if this proxy is 10.10.1.100 on some random port, and the gateway to ppp1 is 10.6.6.6, issue the following:
route add 10.10.1.100 gw 10.6.6.6