when you say subtract, do you mean you want to delete the offending line, or do you want to clip away only that part that is offending?
If you want to get rid of the line, then a simple query like this would suffice:
FROM a, b
I tested it, and that of course finds the lines that are actually touching one another. But, I also created two lines that are within 3m of one another. If you change the precision to 5m, then you won't have to do any kind of buffering, as it will select those lines since they are within the tolerance.
Is that what you are looking to do? Or, do you need to clip out the part of the line that is coincident with the other line?