mix two lists

Hi everybody!

i have a question for you guys... I have two lists of segment of a polyline.The two lists contain the segments that go in opposite directions and i need just one list ordinate that mix one by one the segment of the first list with those of the second one. So i can get a polyline with a ordinate segments inside. Hope you can help me. I know it shouldn't be so hard! I attach here the screenshot. thankssss.