One way is to reflect the image and then combine with the original, e.g., by multiplying the two images.
img = Import["https://i.sstatic.net/26xqd6nM.jpg"];
imgR = ImageReflect[img];
ImageMultiply[img,imgR]
which gives:
This works because the two curves you're interested in are equally spaced vertically.
If you'd like to remove the other curves from the image to make it less busy, you could use DominantColors[img]
to identify the colors and ColorDetect[img,color]
to extract a mask of each curve using its color as returned by DominantColors. Then combine these masks with the original image to get an image with just the two curves you want.