You tell the fax scheduler about the two types of lines by putting them into different Trunk Groups in the Fax Line Database.
In the Fax Line Database, you should have your two fax lines, probably fax-line-1 and fax-line-2, although they may be named anything you want. Suppose fax-line-1 is the one that uses the Device file /dev/ttyd00, which usually is the one for Port A, and you have the fax modem on that port connected to the regular line. Likewise, suppose fax-line-2 has its modem connected to the leased line. You would then want to put the two into two different Trunk Groups. You do this by giving fax-line-1 a Trunk Group Name of, say, "Regular", and fax-line-2, say, "Leased". If you have more modems, you group them under the same Trunk Group by giving them the same Trunk Group Name if they are connected to the same type of phone lines.