Most ISDN cards/TA's have an AT command that will let you dial out on two channels. This can be put into the /etc/e-smith/templates/etc/sysconfig/network-scripts/chat-ppp0/25init template file. I've seen several versions of this, but the most common seems to be ATDI2 (or 3 or 4, depending on the card). Check the card docs to see if you can do it this way.
Richard