joining two surfaces

hey guys please consider that i have two faces , and i want to join them by a curve , now my question is how should i do it so that the curve must be colinear to two faces at the same time , this condidtion is very important for me . i used loft and i also use sweep multi and splines with a mid point , i always use center of my faces as my points for them to join via sweep or Loft or .... , i really appreciate your help