From a one-faced line, where all the dancers are facing the same way, the two couples will be turning in opposite directions. The couple on the left will be turning to the right and the couple on the right will be turning to the left.
From a two-faced line, where the two couples are facing opposite directions, the two couples will turn in the same direction. For example, from a right-handed two-faced line both couples will be turning to the right.
But in either case, the couples that made up the original line become facing couples.
It is very common for two parallel lines to do a Bend the Line at the same time. Each line becomes a pair of facing couples, and the result for the entire formation will be facing lines. This will be the result regardless of whether the original lines were one-faced or two-faced.