I took the lazy route to solve this problem by using s_title and b_title (which I commented out in their usual locations) as new text entry variables for 2nd shipping and billing address lines. It works, but I've yet to crack the problem cited by xcell67 above. X-Cart is filling in s_title with the value in b_title when s_title is missing, and my newbie attempts to write logic to to prevent this have failed.
Question: When I go back to modify address info as entered from my site after submitting it, X-Cart has filled in the missing shipping variables with what it finds in the billing variables. In what part of the code is this substitution taking place? Anyone know how to modify that code to pull off the type of conditional substitution described by xcell67?
And X-Cart developers, if you're reading, I love your product but this is a highly needed feature.